Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 Investment		Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 6,090,676	[1]	$ 5,978,236
Less than 12 months - Unrealized Losses	(18,631)	[1]	(152,383)
12 months or longer - Unrealized Losses	1,023,081	[1]	154,250
12 months or longer - Unrealized Losses	(28,219)	[1]	(8,015)
Fair Value of Investments	7,113,757	[1]	6,132,486
Unrealized Losses	(46,850)	[1]	(160,398)
Number of investments in unrealized loss position	182
Number of investments with unrealized losses greater than 12 months	34
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	4,736,436
Less than 12 months - Unrealized Losses	(12,898)
12 months or longer - Unrealized Losses	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	4,736,436
Unrealized Losses	(12,898)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	688,808		1,821,045
Less than 12 months - Unrealized Losses	(2,373)		(40,785)
12 months or longer - Unrealized Losses	553,231		0
12 months or longer - Unrealized Losses	(12,667)		0
Fair Value of Investments	1,242,039		1,821,045
Unrealized Losses	(15,040)		(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	659,886		2,098,137
Less than 12 months - Unrealized Losses	(2,523)		(79,519)
12 months or longer - Unrealized Losses	469,850		134,420
12 months or longer - Unrealized Losses	(15,552)		(5,903)
Fair Value of Investments	1,129,736		2,232,557
Unrealized Losses	(18,075)		(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			109,699
Less than 12 months - Unrealized Losses			(123)
12 months or longer - Unrealized Losses			0
12 months or longer - Unrealized Losses			0
Fair Value of Investments			109,699
Unrealized Losses			(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	5,546		910
Less than 12 months - Unrealized Losses	(837)		(483)
12 months or longer - Unrealized Losses	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	5,546		910
Unrealized Losses	$ (837)		$ (483)

[1] As of September 30, 2014, we identified a total of 182 investments that were in unrealized loss positions, of which 34 investments totaling $1.0 billion with unrealized losses of $28.2 million have been in an impaired position for a period of time greater than 12 months. As of September 30, 2014, we do not intend to sell any impaired fixed income investment 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 September 30, 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.