Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Mar. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)	Mar. 31, 2016 Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 2,916,041	$ 2,290,037
Less than 12 months - Unrealized Losses	(11,699)	(10,896)
12 months or longer - Fair Value of Investments	262,971	233,548
12 months or longer - Unrealized Losses	(6,484)	(6,721)
Fair Value of Investments	3,179,012	2,523,585
Unrealized Losses	$ (18,183)	(17,617)
Number of investments in unrealized loss position \| Investment	189		174
Number of investments with unrealized losses greater than 12 months \| Investment	36		20
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 1,377,573	879,255
Less than 12 months - Unrealized Losses	(1,454)	(1,190)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	1,377,573	879,255
Unrealized Losses	(1,454)	(1,190)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	513,209	513,198
Less than 12 months - Unrealized Losses	(1,457)	(1,603)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	513,209	513,198
Unrealized Losses	(1,457)	(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	843,542	635,566
Less than 12 months - Unrealized Losses	(7,607)	(6,704)
12 months or longer - Fair Value of Investments	214,507	227,480
12 months or longer - Unrealized Losses	(6,276)	(6,694)
Fair Value of Investments	1,058,049	863,046
Unrealized Losses	(13,883)	(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	179,675	258,325
Less than 12 months - Unrealized Losses	(454)	(613)
12 months or longer - Fair Value of Investments	48,464	6,068
12 months or longer - Unrealized Losses	(208)	(27)
Fair Value of Investments	228,139	264,393
Unrealized Losses	(662)	(640)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,042	3,693
Less than 12 months - Unrealized Losses	(727)	(786)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	2,042	3,693
Unrealized Losses	$ (727)	$ (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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