Financial Instruments - Derivative Instruments Notional Amount Outstanding (Detail) - Non-Deal Contingent Swaps - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Foreign currency contracts
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	$ 411,788	$ 896,947
Interest rate swaps
Schedule Of Information By Major Category Of Credit Derivatives Contracts [Line Items]
Derivative instruments outstanding	$ 0	$ 150,000