Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Amortized Cost	$ 12,514,893	$ 16,375,941
Unrealized Gains	154,075	47,578
Unrealized Losses	(3,271)	(42,771)
Carrying Value	12,665,697	16,380,748
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	8,828,906	11,679,450
Unrealized Gains	110,721	19,134
Unrealized Losses	0	(20,549)
Carrying Value	8,939,627	11,678,035
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	2,089,590	2,677,453
Unrealized Gains	36,982	17,684
Unrealized Losses	0	(5,108)
Carrying Value	2,126,572	2,690,029
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	1,089,956	1,408,206
Unrealized Gains	4,896	6,591
Unrealized Losses	(2,563)	(15,518)
Carrying Value	1,092,289	1,399,279
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	504,134	604,236
Unrealized Gains	839	3,709
Unrealized Losses	(486)	(9)
Carrying Value	504,487	607,936
Equity securities
Investment Holdings [Line Items]
Amortized Cost	2,307	6,596
Unrealized Gains	637	460
Unrealized Losses	(222)	(1,587)
Carrying Value	$ 2,722	$ 5,469

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
-Paragraph 5
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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
-SubTopic 10
-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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