Derivatives and Hedging Activities - Outstanding Interest Rate Swaptions and Derivative Instrument Information (Detail)	9 Months Ended
	Sep. 30, 2017 USD ($)
Derivative Interest Rate Swaptions
Notional Amount as of January 1, 2017	$ 7,132,274,000
Additions	2,827,468,000
Settlement, Termination, Expiration or Exercise	(210,893,000)
Notional Amount as of September 30, 2017	9,748,849,000
Interest Rate Swaps
Derivative Interest Rate Swaptions
Notional Amount as of January 1, 2017	6,500,000,000
Additions	2,620,000,000
Settlement, Termination, Expiration or Exercise	0
Notional Amount as of September 30, 2017	9,120,000,000
Currency Forward Contracts
Derivative Interest Rate Swaptions
Notional Amount as of January 1, 2017	62,308,000
Additions	207,468,000
Settlement, Termination, Expiration or Exercise	(200,360,000)
Notional Amount as of September 30, 2017	69,416,000
Credit Derivatives
Derivative Interest Rate Swaptions
Notional Amount as of January 1, 2017	569,966,000
Additions	0
Settlement, Termination, Expiration or Exercise	(10,533,000)
Notional Amount as of September 30, 2017	$ 559,433,000