Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Mar. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 8,539,716	$ 8,913,199
Less than 12 months - Unrealized Losses	(102,098)	(51,432)
12 months or longer - Fair Value of Investments	1,082,370	713,348
12 months or longer - Unrealized Losses	(8,382)	(5,752)
Fair Value of Investments	9,622,086	9,626,547
Unrealized Losses	$ (110,480)	$ (57,184)
Number of investments in unrealized loss position | Investment	281	268
Number of investments with unrealized losses greater than 12 months | Investment	52	46
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 5,319,620	$ 5,968,914
Less than 12 months - Unrealized Losses	(45,520)	(23,397)
12 months or longer - Fair Value of Investments	697,257	323,966
12 months or longer - Unrealized Losses	(4,000)	(2,282)
Fair Value of Investments	6,016,877	6,292,880
Unrealized Losses	(49,520)	(25,679)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,079,618	736,541
Less than 12 months - Unrealized Losses	(4,168)	(2,289)
12 months or longer - Fair Value of Investments	335,109	336,196
12 months or longer - Unrealized Losses	(4,262)	(3,347)
Fair Value of Investments	1,414,727	1,072,737
Unrealized Losses	(8,430)	(5,636)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Investments, Unrealized Loss Position [Line Items]
Fair Value of Investments	2,118,967	2,193,277
Unrealized Losses	(52,403)	(25,534)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Investments, Unrealized Loss Position [Line Items]
Fair Value of Investments	71,515	67,029
Unrealized Losses	(127)	(126)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		624
Less than 12 months - Unrealized Losses		(209)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		624
Unrealized Losses		(209)
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,118,967	2,193,277
Less than 12 months - Unrealized Losses	(52,403)	(25,534)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Variable rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	21,511	13,843
Less than 12 months - Unrealized Losses	(7)	(3)
12 months or longer - Fair Value of Investments	50,004	53,186
12 months or longer - Unrealized Losses	$ (120)	$ (123)