Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 218,104	$ 1,681,127
Less than 12 months - Unrealized Losses	(842)	(5,800)
12 months or longer - Fair Value of Investments	250	0
12 months or longer - Unrealized Losses	(194)	0
Fair Value of Investments	218,354	1,681,127
Unrealized Losses	(1,036)	(5,800)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		50,994
Less than 12 months - Unrealized Losses		(90)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		50,994
Unrealized Losses		(90)
Residential mortgage-backed securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		54,588
Less than 12 months - Unrealized Losses		(106)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		54,588
Unrealized Losses		(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	205,942	50,125
Less than 12 months - Unrealized Losses	(742)	(71)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	205,942	50,125
Unrealized Losses	(742)	(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	10,611	1,521,589
Less than 12 months - Unrealized Losses	(5)	(4,334)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	10,611	1,521,589
Unrealized Losses	(5)	(4,334)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,551	3,831
Less than 12 months - Unrealized Losses	(95)	(1,199)
12 months or longer - Fair Value of Investments	250	0
12 months or longer - Unrealized Losses	(194)	0
Fair Value of Investments	1,801	3,831
Unrealized Losses	$ (289)	$ (1,199)