Annual report pursuant to Section 13 and 15(d)

Fair Value Measures (Narrative) (Details)

v3.10.0.1
Fair Value Measures (Narrative) (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
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
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
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
Cash Equivalents [Member] | Repurchase Agreements [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   4,995
Cash Equivalents [Member] | Repurchase Agreements [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
Cash Equivalents [Member] | Repurchase Agreements [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
Cash Equivalents [Member] | Repurchase Agreements [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   4,995
Available-for-sale Securities [Member] | Variable Rate Demand Obligation [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   19,963
Available-for-sale Securities [Member] | Variable Rate Demand Obligation [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
Available-for-sale Securities [Member] | Variable Rate Demand Obligation [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   0
Available-for-sale Securities [Member] | Variable Rate Demand Obligation [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure   $ 19,963