Derivatives and Hedge Accounting - Additional Information (Details) £ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative losses which could be reclassified into earnings within the next 12 months			$ 6,839,000
Maximum outstanding term of cash flow hedges (in months)			45 months
Gain/(losses) that reclassified from AOCI into earning for discontinued hedging transactions	$ 0	$ 0	$ 0	$ 0
Derivatives Designated as Hedging Instruments | Derivatives in Cash Flow Hedging Relationships | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts outstanding	297,643,000		297,643,000			$ 218,545,000
Derivatives Designated as Hedging Instruments | Derivatives in Cash Flow Hedging Relationships | Foreign Exchange Range Forward Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts outstanding	1,500,000		1,500,000
Derivatives not Designated as Hedging Instruments | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts outstanding	$ 91,523,000		$ 91,523,000		£ 17,244	$ 63,980,000	£ 17,974