Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 957,319	$ 8,913,199
Less than 12 months - Unrealized Losses	(5,412)	(51,432)
12 months or longer - Fair Value of Investments	6,039,639	713,348
12 months or longer - Unrealized Losses	(78,938)	(5,752)
Fair Value of Investments	6,996,958	9,626,547
Unrealized Losses	$ (84,350)	$ (57,184)
Number of investments in unrealized loss position | Investment	200	268
Number of investments with unrealized losses greater than 12 months | Investment	162	46
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 494,287	$ 5,968,914
Less than 12 months - Unrealized Losses	(3,785)	(23,397)
12 months or longer - Fair Value of Investments	3,568,119	323,966
12 months or longer - Unrealized Losses	(31,777)	(2,282)
Fair Value of Investments	4,062,406	6,292,880
Unrealized Losses	(35,562)	(25,679)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	443,790	736,541
Less than 12 months - Unrealized Losses	(1,602)	(2,289)
12 months or longer - Fair Value of Investments	591,216	336,196
12 months or longer - Unrealized Losses	(4,768)	(3,347)
Fair Value of Investments	1,035,006	1,072,737
Unrealized Losses	(6,370)	(5,636)
Foreign government debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	5,812
Less than 12 months - Unrealized Losses	(3)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	5,812
Unrealized Losses	(3)
Residential mortgage-backed securities | Fixed rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	13,430	2,193,277
Less than 12 months - Unrealized Losses	(22)	(25,534)
12 months or longer - Fair Value of Investments	1,866,788	0
12 months or longer - Unrealized Losses	(42,378)	0
Fair Value of Investments	1,880,218	2,193,277
Unrealized Losses	(42,400)	(25,534)
Residential mortgage-backed securities | Variable rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	0	13,843
Less than 12 months - Unrealized Losses	0	(3)
12 months or longer - Fair Value of Investments	13,516	53,186
12 months or longer - Unrealized Losses	(15)	(123)
Fair Value of Investments	13,516	67,029
Unrealized Losses	$ (15)	(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)