Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 15,167,233	$ 13,497,945
Unrealized Gains	149,059	70,853
Unrealized Losses	(8,631)	(28,143)
Carrying Value	15,307,661	13,540,655
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	10,103,580	7,289,135
Unrealized Gains	93,458	17,524
Unrealized Losses	(13)	(4,386)
Carrying Value	10,197,025	7,302,273
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	2,902,479	3,540,055
Unrealized Gains	40,502	30,478
Unrealized Losses	(13)	(8,977)
Carrying Value	2,942,968	3,561,556
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	1,513,691	1,884,450
Unrealized Gains	10,652	14,851
Unrealized Losses	(6,641)	(14,458)
Carrying Value	1,517,702	1,884,843
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	641,057	779,103
Unrealized Gains	4,231	5,372
Unrealized Losses	0	0
Carrying Value	645,288	784,475
Equity securities
Investment Holdings [Line Items]
Amortized Cost	6,426	5,202
Unrealized Gains	216	2,628
Unrealized Losses	(1,964)	(322)
Carrying Value	$ 4,678	$ 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

-Topic 320

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

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

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