Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 2,997,689	[1]	$ 1,681,127
Less than 12 months - Unrealized Losses	(13,298)	[1]	(5,800)
12 months or longer - Fair Value of Investments	44,288	[1]
12 months or longer - Unrealized Losses	(45)	[1]
Fair Value of Investments	3,041,977	[1]	1,681,127
Unrealized Losses	(13,343)	[1]	(5,800)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	326,485		50,994
Less than 12 months - Unrealized Losses	(1,112)		(90)
Fair Value of Investments	326,485		50,994
Unrealized Losses	(1,112)		(90)
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	359,773		54,588
Less than 12 months - Unrealized Losses	(2,982)		(106)
Fair Value of Investments	359,773		54,588
Unrealized Losses	(2,982)		(106)
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	854,316		50,125
Less than 12 months - Unrealized Losses	(5,374)		(71)
Fair Value of Investments	854,316		50,125
Unrealized Losses	(5,374)		(71)
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	1,455,913		1,521,589
Less than 12 months - Unrealized Losses	(2,961)		(4,334)
12 months or longer - Fair Value of Investments	44,288
12 months or longer - Unrealized Losses	(45)
Fair Value of Investments	1,500,201		1,521,589
Unrealized Losses	(3,006)		(4,334)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,202		3,831
Less than 12 months - Unrealized Losses	(869)		(1,199)
Fair Value of Investments	1,202		3,831
Unrealized Losses	$ (869)		$ (1,199)

[1] As of March 31, 2012, we identified a total of 143 investments that were in unrealized loss positions, of which one investment totaling $44.3 million with unrealized losses of $45 thousand has been in an impaired position for a period of time greater than 12 months. As of March 31, 2012, we do not intend to sell any impaired 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 debt securities prior to recovery of our adjusted cost basis. Based on our analysis as of March 31, 2012, 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.