Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Sep. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 302,363	$ 3,209,821
Less than 12 months - Unrealized Losses	(2,455)	(8,301)
12 months or longer - Unrealized Losses	396,781	961,186
12 months or longer - Unrealized Losses	(6,176)	(19,842)
Fair Value of Investments	699,144	4,171,007
Unrealized Losses	$ (8,631)	$ (28,143)
Number of investments in unrealized loss position | Investment	43	115
Number of investments with unrealized losses greater than 12 months | Investment	18	33
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 51,576	$ 2,297,895
Less than 12 months - Unrealized Losses	(13)	(4,386)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	51,576	2,297,895
Unrealized Losses	(13)	(4,386)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	17,424	249,266
Less than 12 months - Unrealized Losses	(13)	(489)
12 months or longer - Unrealized Losses	0	507,385
12 months or longer - Unrealized Losses	0	(8,488)
Fair Value of Investments	17,424	756,651
Unrealized Losses	(13)	(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	230,141	662,092
Less than 12 months - Unrealized Losses	(465)	(3,104)
12 months or longer - Unrealized Losses	396,781	453,801
12 months or longer - Unrealized Losses	(6,176)	(11,354)
Fair Value of Investments	626,922	1,115,893
Unrealized Losses	(6,641)	(14,458)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	3,222	568
Less than 12 months - Unrealized Losses	$ (1,964)	(322)
12 months or longer - Unrealized Losses		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments	$ 3,222	568
Unrealized Losses	$ (1,964)	$ (322)