Fair Value Measurements (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Debt, Fair Value Disclosures	$ 562,600,000	$ 503,700,000
Convertible Debt	377,863,000	369,259,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	41,399,000	36,574,000
Foreign Currency Contracts, Liability, Fair Value Disclosure	(25,829,000)	(22,495,000)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	41,399,000	36,574,000
Foreign Currency Contracts, Liability, Fair Value Disclosure	(25,829,000)	(22,495,000)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign Currency Contract, Asset, Fair Value Disclosure	0	0
Foreign Currency Contracts, Liability, Fair Value Disclosure	$ 0	$ 0