Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amortized cost	$ 7,862,311	$ 11,131,008
Unrealized Gains	12,082	46,840
Unrealized Losses	(84,350)	(57,184)
Carrying Value	7,790,043	11,120,664
U.S. treasury securities
Investment Holdings [Line Items]
Amortized cost	4,762,182	6,865,068
Unrealized Gains	11,638	1,113
Unrealized Losses	(35,562)	(25,679)
Carrying Value	4,738,258	6,840,502
U.S. agency debentures
Investment Holdings [Line Items]
Amortized cost	1,090,426	1,569,195
Unrealized Gains	61	3,569
Unrealized Losses	(6,370)	(5,636)
Carrying Value	1,084,117	1,567,128
Foreign government debt securities
Investment Holdings [Line Items]
Amortized cost	5,815
Unrealized Gains	0
Unrealized Losses	(3)
Carrying Value	5,812
Residential mortgage-backed securities \| Fixed rate
Investment Holdings [Line Items]
Amortized cost	1,922,618	2,292,311
Unrealized Gains	0	258
Unrealized Losses	(42,400)	(25,534)
Carrying Value	1,880,218	2,267,035
Residential mortgage-backed securities \| Variable rate
Investment Holdings [Line Items]
Amortized cost	81,270	372,481
Unrealized Gains	383	1,375
Unrealized Losses	(15)	(126)
Carrying Value	$ 81,638	373,730
Equity securities
Investment Holdings [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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