Investment Securities - Summary of Unrealized Losses on Available for Sale Securities (Detail) $ in Thousands	Jun. 30, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	$ 187,616	$ 8,783,725
Less than 12 months - Unrealized Losses	(408)	(31,934)
12 months or longer - Fair Value of Investments	263,702	373,284
12 months or longer - Unrealized Losses	(1,910)	(10,837)
Fair Value of Investments	451,318	9,157,009
Unrealized Losses	$ (2,318)	$ (42,771)
Number of investments in unrealized loss position \| Investment	74	243
Number of investments with unrealized losses greater than 12 months \| Investment	14	18
U.S. treasury securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		$ 7,467,519
Less than 12 months - Unrealized Losses		(20,549)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		7,467,519
Unrealized Losses		(20,549)
U.S. agency debentures
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments		760,071
Less than 12 months - Unrealized Losses		(5,108)
12 months or longer - Fair Value of Investments		0
12 months or longer - Unrealized Losses		0
Fair Value of Investments		760,071
Unrealized Losses		(5,108)
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	$ 0	545,404
Less than 12 months - Unrealized Losses	0	(4,681)
12 months or longer - Fair Value of Investments	263,702	373,284
12 months or longer - Unrealized Losses	(1,910)	(10,837)
Fair Value of Investments	263,702	918,688
Unrealized Losses	(1,910)	(15,518)
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	187,429	7,776
Less than 12 months - Unrealized Losses	(348)	(9)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	187,429	7,776
Unrealized Losses	(348)	(9)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months - Fair Value of Investments	187	2,955
Less than 12 months - Unrealized Losses	(60)	(1,587)
12 months or longer - Fair Value of Investments	0	0
12 months or longer - Unrealized Losses	0	0
Fair Value of Investments	187	2,955
Unrealized Losses	$ (60)	$ (1,587)

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
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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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