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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets. fair value $ 903,709 $ 633,110
Book value 28,043 19,009
Held-to-maturity Securities | Short-term investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets. fair value 474,205 206,161
Unrealized gain (loss) (217) $ (96)
Held-to-maturity Securities | Long-term investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets. fair value 14,738  
Unrealized gain (loss) 118  
Book value $ 14,620