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Financial Instruments - Cash and Cash Equivalents (Details) - Fair value - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 458,690 $ 414,111
Fair Value, Inputs, Level 2 | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 287,533 267,236
Fair Value, Inputs, Level 2 | Interest-bearing bank deposits    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 171,157 146,875
Fair Value, Inputs, Level 1 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 0 $ 0