Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,681,127	[1]	$ 2,296,507
Less than 12 months - Unrealized Losses	(5,800)	[1]	(20,396)
12 months or longer - Fair Value of Investments			3,451
12 months or longer - Unrealized Losses			(72)
Fair Value of Investments	1,681,127	[1]	2,299,958
Unrealized Losses	(5,800)	[1]	(20,468)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	50,994		1,731,639
Less than 12 months - Unrealized Losses	(90)		(16,957)
Fair Value of Investments	50,994		1,731,639
Unrealized Losses	(90)		(16,957)
Residential mortgage-backed securities | Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	54,588		32,595
Less than 12 months - Unrealized Losses	(106)		(1,097)
Fair Value of Investments	54,588		32,595
Unrealized Losses	(106)		(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	50,125		322
Less than 12 months - Unrealized Losses	(71)		(1)
Fair Value of Investments	50,125		322
Unrealized Losses	(71)		(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	1,521,589		506,104
Less than 12 months - Unrealized Losses	(4,334)		(1,439)
Fair Value of Investments	1,521,589		506,104
Unrealized Losses	(4,334)		(1,439)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	3,831		148
Less than 12 months - Unrealized Losses	(1,199)		(9)
Fair Value of Investments	3,831		148
Unrealized Losses	(1,199)		(9)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments			25,699
Less than 12 months - Unrealized Losses			(893)
12 months or longer - Fair Value of Investments			3,451
12 months or longer - Unrealized Losses			(72)
Fair Value of Investments			29,150
Unrealized Losses			$ (965)

[1] As of December 31, 2011, we identified a total of 106 investments that were in unrealized loss positions, none of which have 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 December 31, 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 December 31, 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 available-for-sale securities portfolio are reviewed and monitored on a quarterly basis.