Derivatives and Hedging Activities - Outstanding Interest Rate Swaptions and Derivative Instrument Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	$ 9,250,352,000
Additions	1,507,837,000
Settlement, Termination, Expiration or Exercise	(647,657,000)
Notional Amount as of March 31, 2018	10,110,532,000
Interest Rate Swaps
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	8,620,000,000
Additions	1,000,000,000
Settlement, Termination, Expiration or Exercise	(350,000,000)
Notional Amount as of March 31, 2018	9,270,000,000
Interest Rate Swaps with Forward Start Dates
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	500,000,000
Notional Amount as of March 31, 2018	1,300,000,000
Futures Contracts
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	0
Additions	430,000,000
Settlement, Termination, Expiration or Exercise	(215,000,000)
Notional Amount as of March 31, 2018	215,000,000
Currency Forward Contracts
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	76,859,000
Additions	77,837,000
Settlement, Termination, Expiration or Exercise	(76,860,000)
Notional Amount as of March 31, 2018	77,836,000
Credit Derivatives
Derivative Interest Rate Swaptions
Notional Amount as of December 31, 2017	553,493,000
Additions	0
Settlement, Termination, Expiration or Exercise	(5,797,000)
Notional Amount as of March 31, 2018	$ 547,696,000