Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 6,238,397	[1]	$ 5,978,236
Less than 12 months - Unrealized Losses	(121,460)	[1]	(152,383)
12 months or longer - Fair Value of Investments	250,877	[1]	154,250
12 months or longer - Unrealized Losses	(15,076)	[1]	(8,015)
Fair Value of Investments	6,489,274	[1]	6,132,486
Unrealized Losses	(136,536)	[1]	(160,398)
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	683,519
Less than 12 months - Unrealized Losses	(4,734)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	683,519
Unrealized Losses	(4,734)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,570,568		1,821,045
Less than 12 months - Unrealized Losses	(26,808)		(40,785)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	1,570,568		1,821,045
Unrealized Losses	(26,808)		(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	1,599,028		1,480,870
Less than 12 months - Unrealized Losses	(6,764)		(14,029)
12 months or longer - Fair Value of Investments	20,049		19,830
12 months or longer - Unrealized Losses	(1,740)		(2,112)
Fair Value of Investments	1,619,077		1,500,700
Unrealized Losses	(8,504)		(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	1,887,167		2,098,137
Less than 12 months - Unrealized Losses	(63,005)		(79,519)
12 months or longer - Fair Value of Investments	139,570		134,420
12 months or longer - Unrealized Losses	(5,659)		(5,903)
Fair Value of Investments	2,026,737		2,232,557
Unrealized Losses	(68,664)		(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	102,546		109,699
Less than 12 months - Unrealized Losses	(55)		(123)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	102,546		109,699
Unrealized Losses	(55)		(123)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	369,831		464,171
Less than 12 months - Unrealized Losses	(10,143)		(17,423)
12 months or longer - Fair Value of Investments	91,258		0
12 months or longer - Unrealized Losses	(7,677)		0
Fair Value of Investments	461,089		464,171
Unrealized Losses	(17,820)		(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 - Fair Value of Investments			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	25,738		910
Less than 12 months - Unrealized Losses	(9,951)		(483)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	25,738		910
Unrealized Losses	$ (9,951)		$ (483)

[1] As of March 31, 2014, we identified a total of 245 investments that were in unrealized loss positions, of which 11 investments totaling $251 million with unrealized losses of $15.1 million have been in an impaired position for a period of time greater than 12 months. As of March 31, 2014, we do not intend to sell any impaired debt securities prior to recovery of our adjusted cost basis, and it is more likely than not that we will not be required to sell any of our securities prior to recovery of our adjusted cost basis. Based on our analysis as of March 31, 2014, we deem all impairments to be temporary, and therefore changes in value for our temporarily impaired securities as of the same date are included in other comprehensive income. Market valuations and impairment analyses on assets in the available-for-sale securities portfolio are reviewed and monitored on a quarterly basis.