Derivatives and Hedging Activities - Outstanding Interest Rate Swaptions (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013
Derivative Interest Rate Swaptions [Roll Forward]
Notional Amount as of January 1, 2013	$ 8,850,000,000
Additions	9,750,000,000
Settlement, Termination, Expiration or Exercise	(4,550,000,000)
Notional Amount as of December 31, 2013	14,050,000,000
Amount of Realized Gain (loss), net on Interest Rate Derivative Instruments	53,926,000
Swaptions
Derivative Interest Rate Swaptions [Roll Forward]
Notional Amount as of January 1, 2013	850,000,000
Additions	4,450,000,000
Settlement, Termination, Expiration or Exercise	(4,150,000,000)
Notional Amount as of December 31, 2013	1,150,000,000
Amount of Realized Gain (loss), net on Interest Rate Derivative Instruments	56,279,000
Interest Rate Swap
Derivative Interest Rate Swaptions [Roll Forward]
Notional Amount as of January 1, 2013	8,000,000,000
Additions	5,100,000,000
Settlement, Termination, Expiration or Exercise	(300,000,000)
Notional Amount as of December 31, 2013	12,800,000,000
Amount of Realized Gain (loss), net on Interest Rate Derivative Instruments	0
Future
Derivative Interest Rate Swaptions [Roll Forward]
Notional Amount as of January 1, 2013	0
Additions	200,000,000
Settlement, Termination, Expiration or Exercise	(100,000,000)
Notional Amount as of December 31, 2013	100,000,000
Amount of Realized Gain (loss), net on Interest Rate Derivative Instruments	(2,353,000)
Options Held
Interest Rate Swaps [Line Items]
Option Cost	23,275,000
Option Fair Value	2,365,000
Average term	2 months 12 days
Options Held | Payer Swaption | Maximum
Interest Rate Swaps [Line Items]
Option Expiration	6 months
Option Cost	23,275,000
Option Fair Value	2,365,000
Average term	2 months 12 days
Options Held | Payer Swaption | Minimum
Interest Rate Swaps [Line Items]
Option Expiration	6 months
Underlying Swap
Interest Rate Swaps [Line Items]
Average term	10 years 4 days
Underlying Swap Notional Amount	1,150,000,000
Underlying Swap Average Fixed Pay Rate	3.51%
Underlying Swap | Payer Swaption | Maximum
Interest Rate Swaps [Line Items]
Average term	10 years 4 days
Underlying Swap Notional Amount	$ 1,150,000,000
Underlying Swap Average Fixed Pay Rate	3.51%
Underlying Swap Average Receive Rate	3M Libor