Investment Securities - Major Components of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 13,619,702	$ 13,497,945
Unrealized Gains	135,455	70,853
Unrealized Losses	(8,234)	(28,143)
Carrying Value	13,746,923us-gaap_AvailableForSaleSecurities	13,540,655us-gaap_AvailableForSaleSecurities
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	7,842,942	7,289,135
Unrealized Gains	71,447	17,524
Unrealized Losses	(264)	(4,386)
Carrying Value	7,914,125	7,302,273
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	3,271,203	3,540,055
Unrealized Gains	43,675	30,478
Unrealized Losses	(1,214)	(8,977)
Carrying Value	3,313,664	3,561,556
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	1,769,610	1,884,450
Unrealized Gains	15,005	14,851
Unrealized Losses	(6,460)	(14,458)
Carrying Value	1,778,155	1,884,843
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	733,999	779,103
Unrealized Gains	5,207	5,372
Unrealized Losses	(3)	0
Carrying Value	739,203	784,475
Equity securities
Investment Holdings [Line Items]
Amortized Cost	1,948	5,202
Unrealized Gains	121	2,628
Unrealized Losses	(293)	(322)
Carrying Value	$ 1,776	$ 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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-Section 50

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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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-Name Accounting Standards Codification

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