Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Amortized Cost	$ 11,131,008	$ 12,588,783
Unrealized Gains	46,840	49,245
Unrealized Losses	(57,184)	(17,617)
Carrying Value	11,120,664	12,620,411
U.S. treasury securities
Investment Holdings [Line Items]
Amortized Cost	6,865,068	8,880,358
Unrealized Gains	1,113	30,323
Unrealized Losses	(25,679)	(1,190)
Carrying Value	6,840,502	8,909,491
U.S. agency debentures
Investment Holdings [Line Items]
Amortized Cost	1,569,195	2,065,535
Unrealized Gains	3,569	14,443
Unrealized Losses	(5,636)	(1,603)
Carrying Value	1,567,128	2,078,375
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Investment Holdings [Line Items]
Amortized Cost	2,292,311	1,163,017
Unrealized Gains	258	3,046
Unrealized Losses	(25,534)	(13,398)
Carrying Value	2,267,035	1,152,665
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Investment Holdings [Line Items]
Amortized Cost	372,481	474,238
Unrealized Gains	1,375	685
Unrealized Losses	(126)	(640)
Carrying Value	373,730	474,283
Equity securities
Investment Holdings [Line Items]
Amortized Cost	31,953	5,635
Unrealized Gains	40,525	748
Unrealized Losses	(209)	(786)
Carrying Value	$ 72,269	$ 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
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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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+ 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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