Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Amortized Cost	$ 12,584,564	$ 12,588,783
Unrealized Gains	42,140	49,245
Unrealized Losses	(23,367)	(17,617)
Carrying Value	12,603,337	12,620,411
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	7,488,126	8,880,358
Unrealized Gains	14,048	30,323
Unrealized Losses	(3,834)	(1,190)
Carrying Value	7,498,340	8,909,491
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	1,668,403	2,065,535
Unrealized Gains	9,759	14,443
Unrealized Losses	(1,993)	(1,603)
Carrying Value	1,676,169	2,078,375
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	3,024,649	1,163,017
Unrealized Gains	4,998	3,046
Unrealized Losses	(16,952)	(13,398)
Carrying Value	3,012,695	1,152,665
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	394,567	474,238
Unrealized Gains	1,442	685
Unrealized Losses	(129)	(640)
Carrying Value	395,880	474,283
Equity securities
Investment Holdings [Line Items]
Amortized Cost	8,819	5,635
Unrealized Gains	11,893	748
Unrealized Losses	(459)	(786)
Carrying Value	$ 20,253	$ 5,597

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=82897076&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=82897076&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=75030427&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=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=82896770&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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