Summary of Unrealized Losses on Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 637,004	$ 1,681,127
Less than 12 months - Unrealized Losses	(1,663)	(5,800)
12 months or longer - Fair Value of Investments	8,163	0
12 months or longer - Unrealized Losses	(270)	0
Fair Value of Investments	645,167	1,681,127
Unrealized Losses	(1,933)	(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	22,831	54,588
Less than 12 months - Unrealized Losses	(107)	(106)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	22,831	54,588
Unrealized Losses	(107)	(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	461,397	50,125
Less than 12 months - Unrealized Losses	(995)	(71)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	461,397	50,125
Unrealized Losses	(995)	(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	0	1,521,589
Less than 12 months - Unrealized Losses	0	(4,334)
12 months or longer - Fair Value of Investments	7,908	0
12 months or longer - Unrealized Losses	(4)	0
Fair Value of Investments	7,908	1,521,589
Unrealized Losses	(4)	(4,334)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	150,581
Less than 12 months - Unrealized Losses	(489)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	150,581
Unrealized Losses	(489)
Municipal bonds and notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,098
Less than 12 months - Unrealized Losses	(11)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	2,098
Unrealized Losses	(11)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	97	3,831
Less than 12 months - Unrealized Losses	(61)	(1,199)
12 months or longer - Fair Value of Investments	255	0
12 months or longer - Unrealized Losses	(266)	0
Fair Value of Investments	352	3,831
Unrealized Losses	$ (327)	$ (1,199)