Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measures (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details)

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Fair Value Measures (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value $ 300,910 $ 458,003 $ 618,669
Total liabilities measured at fair value 2,230 1,018 3,222
Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 143,840 132,912 217,056
Total liabilities measured at fair value 0 0 0
Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 157,070 325,091 401,613
Total liabilities measured at fair value 2,230 1,018 3,222
Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Total liabilities measured at fair value 0 0 0
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 2,090 1,017 2,895
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 0 0 0
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 2,090 1,017 2,895
Accrued Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 0 0 0
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 140 1 327
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 0 0 0
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 140 1 327
Other Long Term Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total liabilities measured at fair value 0 0 0
Cash Equivalents [Member] | Money Market Funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 130,097    
Cash Equivalents [Member] | Money Market Funds [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 130,097    
Cash Equivalents [Member] | Money Market Funds [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0    
Cash Equivalents [Member] | Money Market Funds [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0    
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 136,721 261,602 225,247
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 136,721 261,602 225,247
Available-For-Sale Short-Term Investments [Member] | Repurchase Agreements [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Available-For-Sale Short-Term Investments [Member] | Municipal Bonds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     37,335
Available-For-Sale Short-Term Investments [Member] | Municipal Bonds [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     0
Available-For-Sale Short-Term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     37,335
Available-For-Sale Short-Term Investments [Member] | Municipal Bonds [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     0
Cash and Cash Equivalents [Member] | Money Market Funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   122,237 144,729
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   122,237 144,729
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   0 0
Cash and Cash Equivalents [Member] | Money Market Funds [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   0 0
Cash and Cash Equivalents [Member] | Time Deposits [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     61,563
Cash and Cash Equivalents [Member] | Time Deposits [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     61,563
Cash and Cash Equivalents [Member] | Time Deposits [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value     0
Cash and Cash Equivalents [Member] | Repurchase Agreements [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   39,952 121,352
Cash and Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   0 0
Cash and Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   39,952 121,352
Cash and Cash Equivalents [Member] | Repurchase Agreements [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value   0 0
Short-term Investments [Member] | Mutual Fund [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 1,477 1,200 1,432
Short-term Investments [Member] | Mutual Fund [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 1,477 1,200 1,432
Short-term Investments [Member] | Mutual Fund [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Short-term Investments [Member] | Mutual Fund [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 13,573 13,615 7,163
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 13,573 13,615 7,163
Other Current Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Noncurrent Assets [Member] | Money Market Funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 1,312 869  
Other Noncurrent Assets [Member] | Money Market Funds [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 1,312 869  
Other Noncurrent Assets [Member] | Money Market Funds [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0  
Other Noncurrent Assets [Member] | Money Market Funds [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0  
Other Noncurrent Assets [Member] | Mutual Fund [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 10,954 8,606 9,332
Other Noncurrent Assets [Member] | Mutual Fund [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 10,954 8,606 9,332
Other Noncurrent Assets [Member] | Mutual Fund [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Noncurrent Assets [Member] | Mutual Fund [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Noncurrent Assets [Member] | Derivative Financial Instruments, Assets [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 6,776 9,922 10,516
Other Noncurrent Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 0 0 0
Other Noncurrent Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value 6,776 9,922 10,516
Other Noncurrent Assets [Member] | Derivative Financial Instruments, Assets [Member] | Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total assets measured at fair value $ 0 $ 0 $ 0