Investment Securities - Major Components of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Amortized Cost	$ 13,322,059	$ 12,055,524
Unrealized Gains	58,227	91,695
Unrealized Losses	(46,850)	(160,398)
Carrying Value	13,333,436	11,986,821
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	6,886,952
Unrealized Gains	2,245
Unrealized Losses	(12,898)
Carrying Value	6,876,299
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	3,592,129	4,344,652
Unrealized Gains	28,744	41,365
Unrealized Losses	(15,040)	(40,785)
Carrying Value	3,605,833	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	2,003,337	3,386,670
Unrealized Gains	20,671	24,510
Unrealized Losses	(18,075)	(85,422)
Carrying Value	2,005,933	3,325,758
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investment Holdings [Line Items]
Amortized Cost	830,139	1,183,333
Unrealized Gains	5,818	3,363
Unrealized Losses	0	(123)
Carrying Value	835,957	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	9,502	4,842
Unrealized Gains	749	692
Unrealized Losses	(837)	(483)
Carrying Value	$ 9,414	$ 5,051