Summary of Unrealized Losses on Available for Sale Investment Securities (Detail) (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,382,469	[1]	$ 2,296,507
Less than 12 months - Unrealized Losses	(2,354)	[1]	(20,396)
12 months or longer - Fair Value of Investments	44,606	[1]	3,451
12 months or longer - Unrealized Losses	(33)	[1]	(72)
Fair Value of Investments	1,427,075	[1]	2,299,958
Unrealized Losses	(2,387)	[1]	(20,468)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	25,000		1,731,639
Less than 12 months - Unrealized Losses	(2)		(16,957)
Fair Value of Investments	25,000		1,731,639
Unrealized Losses	(2)		(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	51,756		32,595
Less than 12 months - Unrealized Losses	(160)		(1,097)
Fair Value of Investments	51,756		32,595
Unrealized Losses	(160)		(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			322
Less than 12 months - Unrealized Losses			(1)
Fair Value of Investments			322
Unrealized Losses			(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	1,304,198		506,104
Less than 12 months - Unrealized Losses	(2,059)		(1,439)
12 months or longer - Fair Value of Investments	44,606
12 months or longer - Unrealized Losses	(33)
Fair Value of Investments	1,348,804		506,104
Unrealized Losses	(2,092)		(1,439)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,195		25,699
Less than 12 months - Unrealized Losses	(5)		(893)
12 months or longer - Fair Value of Investments			3,451
12 months or longer - Unrealized Losses			(72)
Fair Value of Investments	1,195		29,150
Unrealized Losses	(5)		(965)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	320		148
Less than 12 months - Unrealized Losses	(128)		(9)
Fair Value of Investments	320		148
Unrealized Losses	$ (128)		$ (9)

[1] As of September 30, 2011, we identified a total of 85 investments that were in unrealized loss positions, of which one investment totaling $44.6 million with unrealized losses of $33 thousand has been in an impaired position for a period of time greater than 12 months. 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 September 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 as of September 30, 2011, we deem all impairments to be temporary and 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 investment securities portfolio are reviewed and monitored on a quarterly basis.