Investment Securities - Summary of Available for Sale Securities (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 8,913,199	$ 2,290,037
Less than 12 months - Unrealized Losses	(51,432)	(10,896)
12 months or longer - Fair Value of Investments	713,348	233,548
12 months or longer - Unrealized Losses	(5,752)	(6,721)
Fair Value of Investments	9,626,547	2,523,585
Unrealized Losses	$ (57,184)	$ (17,617)
Number of investments in unrealized loss position \| Investment	268	174
Number of investments with unrealized losses greater than 12 months \| Investment	46	20
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 5,968,914	$ 879,255
Less than 12 months - Unrealized Losses	(23,397)	(1,190)
12 months or longer - Fair Value of Investments	323,966	0
12 months or longer - Unrealized Losses	(2,282)	0
Fair Value of Investments	6,292,880	879,255
Unrealized Losses	(25,679)	(1,190)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	736,541	513,198
Less than 12 months - Unrealized Losses	(2,289)	(1,603)
12 months or longer - Fair Value of Investments	336,196	0
12 months or longer - Unrealized Losses	(3,347)	0
Fair Value of Investments	1,072,737	513,198
Unrealized Losses	(5,636)	(1,603)
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	2,193,277	635,566
Less than 12 months - Unrealized Losses	(25,534)	(6,704)
12 months or longer - Fair Value of Investments	0	227,480
12 months or longer - Unrealized Losses	0	(6,694)
Fair Value of Investments	2,193,277	863,046
Unrealized Losses	(25,534)	(13,398)
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	13,843	258,325
Less than 12 months - Unrealized Losses	(3)	(613)
12 months or longer - Fair Value of Investments	53,186	6,068
12 months or longer - Unrealized Losses	(123)	(27)
Fair Value of Investments	67,029	264,393
Unrealized Losses	(126)	(640)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	624	3,693
Less than 12 months - Unrealized Losses	(209)	(786)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	624	3,693
Unrealized Losses	$ (209)	$ (786)

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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.

+ References

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-Publisher FASB
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