Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Jun. 30, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)	Jun. 30, 2016 Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 4,349,010	$ 2,290,037
Less than 12 months - Unrealized Losses	(13,688)	(10,896)
12 months or longer - Fair Value of Investments	271,692	233,548
12 months or longer - Unrealized Losses	(6,104)	(6,721)
Fair Value of Investments	4,620,702	2,523,585
Unrealized Losses	$ (19,792)	(17,617)
Number of investments in unrealized loss position \| Investment	184		174
Number of investments with unrealized losses greater than 12 months \| Investment	44		20
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 2,724,519	879,255
Less than 12 months - Unrealized Losses	(3,599)	(1,190)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	2,724,519	879,255
Unrealized Losses	(3,599)	(1,190)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	762,920	513,198
Less than 12 months - Unrealized Losses	(1,577)	(1,603)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	762,920	513,198
Unrealized Losses	(1,577)	(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	851,825	635,566
Less than 12 months - Unrealized Losses	(7,849)	(6,704)
12 months or longer - Fair Value of Investments	201,986	227,480
12 months or longer - Unrealized Losses	(5,971)	(6,694)
Fair Value of Investments	1,053,811	863,046
Unrealized Losses	(13,820)	(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	6,185	258,325
Less than 12 months - Unrealized Losses	(14)	(613)
12 months or longer - Fair Value of Investments	69,706	6,068
12 months or longer - Unrealized Losses	(133)	(27)
Fair Value of Investments	75,891	264,393
Unrealized Losses	(147)	(640)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	3,561	3,693
Less than 12 months - Unrealized Losses	(649)	(786)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	3,561	3,693
Unrealized Losses	$ (649)	$ (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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