Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Dec. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 2,102,365	$ 957,319
Less than 12 months - Unrealized Losses	(20,007)	(5,412)
12 months or longer - Fair Value of Investments	449,850	6,039,639
12 months or longer - Unrealized Losses	(135)	(78,938)
Fair Value of Investments	2,552,215	6,996,958
Unrealized Losses	$ (20,142)	$ (84,350)
Number of investments in unrealized loss position \| Investment	58	200
Number of investments with unrealized losses greater than 12 months \| Investment	12	162
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 971,572	$ 494,287
Less than 12 months - Unrealized Losses	(3,996)	(3,785)
12 months or longer - Fair Value of Investments	449,850	3,568,119
12 months or longer - Unrealized Losses	(135)	(31,777)
Fair Value of Investments	1,421,422	4,062,406
Unrealized Losses	(4,131)	(35,562)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	99,547	443,790
Less than 12 months - Unrealized Losses	(453)	(1,602)
12 months or longer - Fair Value of Investments	0	591,216
12 months or longer - Unrealized Losses	0	(4,768)
Fair Value of Investments	99,547	1,035,006
Unrealized Losses	(453)	(6,370)
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 \| Agency-issued mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	4,014
Less than 12 months - Unrealized Losses	(19)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	4,014
Unrealized Losses	(19)
Residential mortgage-backed securities \| Fixed rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		13,430
Less than 12 months - Unrealized Losses		(22)
12 months or longer - Fair Value of Investments		1,866,788
12 months or longer - Unrealized Losses		(42,378)
Fair Value of Investments		1,880,218
Unrealized Losses		(42,400)
Residential mortgage-backed securities \| Variable rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		0
Less than 12 months - Unrealized Losses		0
12 months or longer - Fair Value of Investments		13,516
12 months or longer - Unrealized Losses		(15)
Fair Value of Investments		13,516
Unrealized Losses		$ (15)
Agency-issued commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,027,232
Less than 12 months - Unrealized Losses	(15,539)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	1,027,232
Unrealized Losses	$ (15,539)

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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-Publisher FASB
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-Topic 320
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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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