Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 680,326	[1]	$ 637,004
Less than 12 months - Unrealized Losses	(3,956)	[1]	(1,663)
12 months or longer - Fair Value of Investments	101,224	[1]	8,163
12 months or longer - Unrealized Losses	(742)	[1]	(270)
Fair Value of Investments	781,550	[1]	645,167
Unrealized Losses	(4,698)	[1]	(1,933)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	303,265
Less than 12 months - Unrealized Losses	(383)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	303,265
Unrealized Losses	(383)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	22,351		22,831
Less than 12 months - Unrealized Losses	(446)		(107)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	22,351		22,831
Unrealized Losses	(446)		(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	252,341		461,397
Less than 12 months - Unrealized Losses	(1,021)		(995)
12 months or longer - Fair Value of Investments	95,676		0
12 months or longer - Unrealized Losses	(501)		0
Fair Value of Investments	348,017		461,397
Unrealized Losses	(1,522)		(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	0		0
Less than 12 months - Unrealized Losses	0		0
12 months or longer - Fair Value of Investments	5,239		7,908
12 months or longer - Unrealized Losses	(28)		(4)
Fair Value of Investments	5,239		7,908
Unrealized Losses	(28)		(4)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	99,554		150,581
Less than 12 months - Unrealized Losses	(1,240)		(489)
12 months or longer - Fair Value of Investments	0		0
12 months or longer - Unrealized Losses	0		0
Fair Value of Investments	99,554		150,581
Unrealized Losses	(1,240)		(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	2,815		97
Less than 12 months - Unrealized Losses	(866)		(61)
12 months or longer - Fair Value of Investments	309		255
12 months or longer - Unrealized Losses	(213)		(266)
Fair Value of Investments	3,124		352
Unrealized Losses	$ (1,079)		$ (327)

[1] As of March 31, 2013, we identified a total of 39 investments that were in unrealized loss positions, of which 9 investments totaling $101.2 million with unrealized losses of $0.7 million have been in an impaired position for a period of time greater than 12 months. As of March 31, 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 March 31, 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.