Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014 Investment	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,117,477	$ 5,978,236
Less than 12 months - Unrealized Losses	(2,900)	(152,383)
12 months or longer - Unrealized Losses	1,035,480	154,250
12 months or longer - Unrealized Losses	(23,362)	(8,015)
Fair Value of Investments	2,152,957	6,132,486
Unrealized Losses	(26,262)	(160,398)
Number of investments in unrealized loss position	81
Number of investments with unrealized losses greater than 12 months	33
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	698,273
Less than 12 months - Unrealized Losses	(823)
12 months or longer - Unrealized Losses	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	698,273
Unrealized Losses	(823)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	200,773	1,821,045
Less than 12 months - Unrealized Losses	(290)	(40,785)
12 months or longer - Unrealized Losses	555,839	0
12 months or longer - Unrealized Losses	(10,182)	0
Fair Value of Investments	756,612	1,821,045
Unrealized Losses	(10,472)	(40,785)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		1,480,870
Less than 12 months - Unrealized Losses		(14,029)
12 months or longer - Unrealized Losses		19,830
12 months or longer - Unrealized Losses		(2,112)
Fair Value of Investments		1,500,700
Unrealized Losses		(16,141)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	135,122	2,098,137
Less than 12 months - Unrealized Losses	(189)	(79,519)
12 months or longer - Unrealized Losses	479,641	134,420
12 months or longer - Unrealized Losses	(13,180)	(5,903)
Fair Value of Investments	614,763	2,232,557
Unrealized Losses	(13,369)	(85,422)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	75,974	109,699
Less than 12 months - Unrealized Losses	(23)	(123)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	75,974	109,699
Unrealized Losses	(23)	(123)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		464,171
Less than 12 months - Unrealized Losses		(17,423)
12 months or longer - Unrealized Losses		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		464,171
Unrealized Losses		(17,423)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		3,404
Less than 12 months - Unrealized Losses		(21)
12 months or longer - Unrealized Losses		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		3,404
Unrealized Losses		(21)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	7,335	910
Less than 12 months - Unrealized Losses	(1,575)	(483)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	7,335	910
Unrealized Losses	$ (1,575)	$ (483)