Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 9,717,156	$ 11,131,008
Unrealized Gains	5,601	46,840
Unrealized Losses	(129,391)	(57,184)
Carrying Value	9,593,366	11,120,664
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,782,879	6,865,068
Unrealized Gains	4,186	1,113
Unrealized Losses	(53,995)	(25,679)
Carrying Value	5,733,070	6,840,502
U.S. agency debentures
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,495,657	1,569,195
Unrealized Gains	0	3,569
Unrealized Losses	(9,793)	(5,636)
Carrying Value	1,485,864	1,567,128
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,103,572	2,292,311
Unrealized Gains	39	258
Unrealized Losses	(65,477)	(25,534)
Carrying Value	2,038,134	2,267,035
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	335,048	372,481
Unrealized Gains	1,376	1,375
Unrealized Losses	(126)	(126)
Carrying Value	$ 336,298	373,730
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		31,953
Unrealized Gains		40,525
Unrealized Losses		(209)
Carrying Value		$ 72,269

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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