Derivatives and Hedge Accounting - Additional Information (Detail) £ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 GBP (£)
Derivative Instruments, Gain (Loss) [Line Items]
Net derivative losses which could be reclassified into earnings within the next 12 months	$ 10,411,000
Maximum outstanding term of cash flow hedges	45 months
Gain/(losses) that reclassified from AOCI into earning for discontinued hedging transactions		$ 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	$ 300,757,000	218,545,000
Derivative not designated as hedging instruments | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts outstanding	$ 98,967,000	$ 63,980,000	£ 17,947	£ 17,974