Annual report pursuant to Section 13 and 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
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure $ 458,003 $ 472,969
Liabilities, Fair Value Disclosure 1,018 14,385
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 132,912 346,425
Liabilities, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 325,091 126,544
Liabilities, Fair Value Disclosure 1,018 14,385
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Liabilities, Fair Value Disclosure 0 0
Money Market Funds [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 122,237 282,860
Money Market Funds [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 869  
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 122,237 282,860
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 869  
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0  
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0  
Bank Time Deposits [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   52,808
Bank Time Deposits [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   52,808
Bank Time Deposits [Member] | Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
Bank Time Deposits [Member] | Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
US Treasury Securities [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 39,952 25,338
US Treasury Securities [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 261,602 73,582
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 39,952 25,338
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 261,602 73,582
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Cash Equivalents [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Derivative Financial Instruments, Assets [Member] | Other Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 13,615 2,331
Derivative Financial Instruments, Assets [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 9,922 335
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 1 [Member] | Other Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 1 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 2 [Member] | Other Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 13,615 2,331
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 2 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 9,922 335
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Other Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Derivative Financial Instruments, Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Mutual Fund Shares [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 1,200 1,438
Mutual Fund Shares [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 8,606 9,319
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 1 [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 1,200 1,438
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 1 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 8,606 9,319
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 2 [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 2 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 3 [Member] | Short-term Investments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Mutual Fund Shares [Member] | Fair Value, Inputs, Level 3 [Member] | Non Current Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure 0 0
Derivative Financial Instruments, Liabilities [Member] | Accrued Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 1,017 10,565
Derivative Financial Instruments, Liabilities [Member] | Other Long Term Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 1 3,820
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 1 [Member] | Accrued Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 0 0
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 1 [Member] | Other Long Term Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 0 0
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 2 [Member] | Accrued Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 1,017 10,565
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 2 [Member] | Other Long Term Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 1 3,820
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 3 [Member] | Accrued Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure 0 0
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 3 [Member] | Other Long Term Liabilities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure $ 0 $ 0