Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 3,472,589	[1]	$ 637,004
Less than 12 months - Unrealized Losses	(81,615)	[1]	(1,663)
12 months or longer - Fair Value of Investments	79,190	[1]	8,163
12 months or longer - Unrealized Losses	(1,633)	[1]	(270)
Fair Value of Investments	3,551,779	[1]	645,167
Unrealized Losses	(83,248)	[1]	(1,933)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	942,337
Less than 12 months - Unrealized Losses	(24,177)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	942,337
Unrealized Losses	(24,177)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	261,787		22,831
Less than 12 months - Unrealized Losses	(2,981)		(107)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	261,787		22,831
Unrealized Losses	(2,981)		(107)
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	1,886,229		461,397
Less than 12 months - Unrealized Losses	(42,555)		(995)
12 months or longer - Fair Value of Investments	79,190		0
12 months or longer - Unrealized Losses	(1,633)		0
Fair Value of Investments	1,965,419		461,397
Unrealized Losses	(44,188)		(995)
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	147,218		0
Less than 12 months - Unrealized Losses	(149)		0
12 months or longer - Fair Value of Investments	0		7,908
12 months or longer - Unrealized Losses	0		(4)
Fair Value of Investments	147,218		7,908
Unrealized Losses	(149)		(4)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	232,474		150,581
Less than 12 months - Unrealized Losses	(10,785)		(489)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	232,474		150,581
Unrealized Losses	(10,785)		(489)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments			2,098
Less than 12 months - Unrealized Losses			(11)
12 months or longer - Fair Value of Investments			0
12 months or longer - Unrealized Losses			0
Fair Value of Investments			2,098
Unrealized Losses			(11)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,544		97
Less than 12 months - Unrealized Losses	(968)		(61)
12 months or longer - Fair Value of Investments	0		255
12 months or longer - Unrealized Losses	0		(266)
Fair Value of Investments	2,544		352
Unrealized Losses	$ (968)		$ (327)

[1] As of September 30, 2013, we identified a total of 134 investments that were in unrealized loss positions, of which 3 investments totaling $79 million with unrealized losses of $1.6 million have been in an impaired position for a period of time greater than 12 months. As of September 30, 2013, we do not intend to sell any impaired debt or equity 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, 2013, 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.