Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Jun. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,443,696	$ 3,209,821
Less than 12 months - Unrealized Losses	(12,002)	(8,301)
12 months or longer - Unrealized Losses	404,495	961,186
12 months or longer - Unrealized Losses	(9,500)	(19,842)
Fair Value of Investments	1,848,191	4,171,007
Unrealized Losses	$ (21,502)	$ (28,143)
Number of investments in unrealized loss position | Investment	94	115
Number of investments with unrealized losses greater than 12 months | Investment	17	33
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 552,547	$ 2,297,895
Less than 12 months - Unrealized Losses	(5,442)	(4,386)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	552,547	2,297,895
Unrealized Losses	(5,442)	(4,386)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	511,643	249,266
Less than 12 months - Unrealized Losses	(3,967)	(489)
12 months or longer - Unrealized Losses	0	507,385
12 months or longer - Unrealized Losses	0	(8,488)
Fair Value of Investments	511,643	756,651
Unrealized Losses	(3,967)	(8,977)
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	375,854	662,092
Less than 12 months - Unrealized Losses	(1,213)	(3,104)
12 months or longer - Unrealized Losses	404,495	453,801
12 months or longer - Unrealized Losses	(9,500)	(11,354)
Fair Value of Investments	780,349	1,115,893
Unrealized Losses	(10,713)	(14,458)
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	933
Less than 12 months - Unrealized Losses	(2)
12 months or longer - Unrealized Losses	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	933
Unrealized Losses	(2)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,719	568
Less than 12 months - Unrealized Losses	(1,378)	(322)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	2,719	568
Unrealized Losses	$ (1,378)	$ (322)