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 |