Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 16,375,941	$ 13,497,945
Unrealized Gains	47,578	70,853
Unrealized Losses	(42,771)	(28,143)
Carrying Value	16,380,748	13,540,655
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	11,679,450	7,289,135
Unrealized Gains	19,134	17,524
Unrealized Losses	(20,549)	(4,386)
Carrying Value	11,678,035	7,302,273
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	2,677,453	3,540,055
Unrealized Gains	17,684	30,478
Unrealized Losses	(5,108)	(8,977)
Carrying Value	2,690,029	3,561,556
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	1,408,206	1,884,450
Unrealized Gains	6,591	14,851
Unrealized Losses	(15,518)	(14,458)
Carrying Value	1,399,279	1,884,843
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	604,236	779,103
Unrealized Gains	3,709	5,372
Unrealized Losses	(9)	0
Carrying Value	607,936	784,475
Equity securities
Investment Holdings [Line Items]
Amortized Cost	6,596	5,202
Unrealized Gains	460	2,628
Unrealized Losses	(1,587)	(322)
Carrying Value	$ 5,469	$ 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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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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
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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