Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 14,414,219	$ 13,497,945
Unrealized Gains	103,042	70,853
Unrealized Losses	(21,502)	(28,143)
Carrying Value	14,495,759	13,540,655
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	8,952,703	7,289,135
Unrealized Gains	54,705	17,524
Unrealized Losses	(5,442)	(4,386)
Carrying Value	9,001,966	7,302,273
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	3,127,635	3,540,055
Unrealized Gains	31,285	30,478
Unrealized Losses	(3,967)	(8,977)
Carrying Value	3,154,953	3,561,556
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	1,641,311	1,884,450
Unrealized Gains	11,995	14,851
Unrealized Losses	(10,713)	(14,458)
Carrying Value	1,642,593	1,884,843
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	687,418	779,103
Unrealized Gains	4,892	5,372
Unrealized Losses	(2)	0
Carrying Value	692,308	784,475
Equity securities
Investment Holdings [Line Items]
Amortized Cost	5,152	5,202
Unrealized Gains	165	2,628
Unrealized Losses	(1,378)	(322)
Carrying Value	$ 3,939	$ 7,508

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

-Paragraph 2

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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