Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Sep. 30, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 4,900,519	$ 2,290,037
Less than 12 months - Unrealized Losses	(13,676)	(10,896)
12 months or longer - Fair Value of Investments	477,708	233,548
12 months or longer - Unrealized Losses	(9,691)	(6,721)
Fair Value of Investments	5,378,227	2,523,585
Unrealized Losses	$ (23,367)	$ (17,617)
Number of investments in unrealized loss position \| Investment	202	174
Number of investments with unrealized losses greater than 12 months \| Investment	63	20
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 2,824,370	$ 879,255
Less than 12 months - Unrealized Losses	(3,834)	(1,190)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	2,824,370	879,255
Unrealized Losses	(3,834)	(1,190)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	562,392	513,198
Less than 12 months - Unrealized Losses	(1,993)	(1,603)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	562,392	513,198
Unrealized Losses	(1,993)	(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	1,505,319	635,566
Less than 12 months - Unrealized Losses	(7,389)	(6,704)
12 months or longer - Fair Value of Investments	415,200	227,480
12 months or longer - Unrealized Losses	(9,563)	(6,694)
Fair Value of Investments	1,920,519	863,046
Unrealized Losses	(16,952)	(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	6,651	258,325
Less than 12 months - Unrealized Losses	(1)	(613)
12 months or longer - Fair Value of Investments	62,508	6,068
12 months or longer - Unrealized Losses	(128)	(27)
Fair Value of Investments	69,159	264,393
Unrealized Losses	(129)	(640)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	1,787	3,693
Less than 12 months - Unrealized Losses	(459)	(786)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	1,787	3,693
Unrealized Losses	$ (459)	$ (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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-Topic 320
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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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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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