Investment Securities - Summary of Available for Sale Securities (Detail) $ in Thousands	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 8,783,725	$ 3,209,821
Less than 12 months - Unrealized Losses	(31,934)	(8,301)
12 months or longer - Fair Value of Investments	373,284	961,186
12 months or longer - Unrealized Losses	(10,837)	(19,842)
Fair Value of Investments	9,157,009	4,171,007
Unrealized Losses	$ (42,771)	$ (28,143)
Number of investments in unrealized loss position \| Investment	243	115
Number of investments with unrealized losses greater than 12 months \| Investment	18	33
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 7,467,519	$ 2,297,895
Less than 12 months - Unrealized Losses	(20,549)	(4,386)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	7,467,519	2,297,895
Unrealized Losses	(20,549)	(4,386)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	760,071	249,266
Less than 12 months - Unrealized Losses	(5,108)	(489)
12 months or longer - Fair Value of Investments	0	507,385
12 months or longer - Unrealized Losses	0	(8,488)
Fair Value of Investments	760,071	756,651
Unrealized Losses	(5,108)	(8,977)
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	545,404	662,092
Less than 12 months - Unrealized Losses	(4,681)	(3,104)
12 months or longer - Fair Value of Investments	373,284	453,801
12 months or longer - Unrealized Losses	(10,837)	(11,354)
Fair Value of Investments	918,688	1,115,893
Unrealized Losses	(15,518)	(14,458)
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	7,776
Less than 12 months - Unrealized Losses	(9)
12 months or longer - Fair Value of Investments	0
12 months or longer - Unrealized Losses	0
Fair Value of Investments	7,776
Unrealized Losses	(9)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	2,955	568
Less than 12 months - Unrealized Losses	(1,587)	(322)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	2,955	568
Unrealized Losses	$ (1,587)	$ (322)

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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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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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