Investment Securities - Major Components of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Amortized Cost	$ 11,613,679	$ 12,055,524
Unrealized Gains	85,373	91,695
Unrealized Losses	(26,262)	(160,398)
Carrying Value	11,672,790	11,986,821
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	4,871,469
Unrealized Gains	11,526
Unrealized Losses	(823)
Carrying Value	4,882,172
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	3,714,346	4,344,652
Unrealized Gains	41,999	41,365
Unrealized Losses	(10,472)	(40,785)
Carrying Value	3,745,873	4,345,232
Agency-issued mortgage-backed securities | Agency-issued collateralized mortgage obligations
Investment Holdings [Line Items]
Amortized Cost		2,472,528
Unrealized Gains		17,189
Unrealized Losses		(16,141)
Carrying Value		2,473,576
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investment Holdings [Line Items]
Amortized Cost	2,128,052	3,386,670
Unrealized Gains	28,146	24,510
Unrealized Losses	(13,369)	(85,422)
Carrying Value	2,142,829	3,325,758
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investment Holdings [Line Items]
Amortized Cost	887,862	1,183,333
Unrealized Gains	3,073	3,363
Unrealized Losses	(23)	(123)
Carrying Value	890,912	1,186,573
Agency-issued commercial mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost		581,475
Unrealized Gains		552
Unrealized Losses		(17,423)
Carrying Value		564,604
Municipal bonds and notes
Investment Holdings [Line Items]
Amortized Cost		82,024
Unrealized Gains		4,024
Unrealized Losses		(21)
Carrying Value		86,027
Equity securities
Investment Holdings [Line Items]
Amortized Cost	11,950	4,842
Unrealized Gains	629	692
Unrealized Losses	(1,575)	(483)
Carrying Value	$ 11,004	$ 5,051