Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Mar. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 483,145	$ 8,783,725
Less than 12 months - Unrealized Losses	(522)	(31,934)
12 months or longer - Unrealized Losses	462,143	373,284
12 months or longer - Unrealized Losses	(4,099)	(10,837)
Fair Value of Investments	945,288	9,157,009
Unrealized Losses	$ (4,621)	$ (42,771)
Number of investments in unrealized loss position | Investment	70	243
Number of investments with unrealized losses greater than 12 months | Investment	24	18
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 201,215	$ 7,467,519
Less than 12 months - Unrealized Losses	(35)	(20,549)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	201,215	7,467,519
Unrealized Losses	(35)	(20,549)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		760,071
Less than 12 months - Unrealized Losses		(5,108)
12 months or longer - Unrealized Losses		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		760,071
Unrealized Losses		(5,108)
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	176,469	545,404
Less than 12 months - Unrealized Losses	(157)	(4,681)
12 months or longer - Unrealized Losses	462,143	373,284
12 months or longer - Unrealized Losses	(4,099)	(10,837)
Fair Value of Investments	638,612	918,688
Unrealized Losses	(4,256)	(15,518)
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	103,900	7,776
Less than 12 months - Unrealized Losses	(127)	(9)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	103,900	7,776
Unrealized Losses	(127)	(9)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,561	2,955
Less than 12 months - Unrealized Losses	(203)	(1,587)
12 months or longer - Unrealized Losses	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	1,561	2,955
Unrealized Losses	$ (203)	$ (1,587)