Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 9,236,301	$ 11,131,008
Unrealized Gains	1,417	46,840
Unrealized Losses	(150,109)	(57,184)
Carrying Value	9,087,609	11,120,664
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,560,828	6,865,068
Unrealized Gains	117	1,113
Unrealized Losses	(59,071)	(25,679)
Carrying Value	5,501,874	6,840,502
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,357,069	1,569,195
Unrealized Gains	0	3,569
Unrealized Losses	(10,397)	(5,636)
Carrying Value	1,346,672	1,567,128
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,001,048	2,292,311
Unrealized Gains	3	258
Unrealized Losses	(80,510)	(25,534)
Carrying Value	1,920,541	2,267,035
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations \| Variable rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	317,356	372,481
Unrealized Gains	1,297	1,375
Unrealized Losses	(131)	(126)
Carrying Value	$ 318,522	373,730
Equity securities
Debt Securities, Available-for-sale [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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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.

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