Summary of Unrealized Losses on Available for Sale Investment Securities (Detail) (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,977,996	[1]	$ 2,296,507
Less than 12 months - Unrealized Losses	(9,168)	[1]	(20,396)
12 months or longer - Fair Value of			3,451
12 months or longer - Unrealized Losses			(72)
Fair Value of Investments	1,977,996	[1]	2,299,958
Unrealized Losses	(9,168)	[1]	(20,468)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	963,408		1,731,639
Less than 12 months - Unrealized Losses	(5,650)		(16,957)
Fair Value of Investments	963,408		1,731,639
Unrealized Losses	(5,650)		(16,957)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	98,038		32,595
Less than 12 months - Unrealized Losses	(1,271)		(1,097)
Fair Value of Investments	98,038		32,595
Unrealized Losses	(1,271)		(1,097)
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	306,876		322
Less than 12 months - Unrealized Losses	(425)		(1)
Fair Value of Investments	306,876		322
Unrealized Losses	(425)		(1)
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	600,587		506,104
Less than 12 months - Unrealized Losses	(1,530)		(1,439)
Fair Value of Investments	600,587		506,104
Unrealized Losses	(1,530)		(1,439)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	8,578		25,699
Less than 12 months - Unrealized Losses	(246)		(893)
12 months or longer - Fair Value of			3,451
12 months or longer - Unrealized Losses			(72)
Fair Value of Investments	8,578		29,150
Unrealized Losses	(246)		(965)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	509		148
Less than 12 months - Unrealized Losses	(46)		(9)
Fair Value of Investments	509		148
Unrealized Losses	$ (46)		$ (9)

[1] As of June 30, 2011, we identified a total of 75 investments that were in unrealized loss positions. Based on the underlying credit quality of the investments, we do not intend to sell any of our securities prior to recovery of our adjusted cost basis and as of June 30, 2011, it is more likely than not that we will not be required to sell any of our debt securities prior to recovery of our adjusted cost basis. Based on our analysis we deem all impairments to be temporary and changes in value for our temporarily impaired securities as of June 30, 2011 are included in other comprehensive income. Market valuations and impairment analyses on assets in the investment securities portfolio are reviewed and monitored on a quarterly basis.