Financial Instruments - Derivative Instruments Notional Amount Outstanding (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Non-Deal Contingent Swaps | Foreign currency contracts
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	$ 531,092	$ 896,947
Non-Deal Contingent Swaps | Interest rate swaps
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	150,000	150,000
Deal Contingent Swaps | Foreign currency contracts
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	1,000,000	0
Deal Contingent Swaps | Interest rate swaps
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	$ 898,513	$ 0