Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Amortized cost	$ 13,894,348	$ 7,862,311
Unrealized Gains	140,713	12,082
Unrealized Losses	(20,142)	(84,350)
Available-for-sale securities	14,014,919	7,790,043
U.S. treasury securities
Investment Holdings [Line Items]
Amortized cost	6,815,874	4,762,182
Unrealized Gains	82,267	11,638
Unrealized Losses	(4,131)	(35,562)
Available-for-sale securities	6,894,010	4,738,258
U.S. agency debentures
Investment Holdings [Line Items]
Amortized cost	100,000	1,090,426
Unrealized Gains	0	61
Unrealized Losses	(453)	(6,370)
Available-for-sale securities	99,547	1,084,117
Foreign government debt securities
Investment Holdings [Line Items]
Amortized cost	9,037	5,815
Unrealized Gains	1	0
Unrealized Losses	0	(3)
Available-for-sale securities	9,038	5,812
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Investment Holdings [Line Items]
Amortized cost	4,109,372
Unrealized Gains	39,438
Unrealized Losses	(19)
Available-for-sale securities	4,148,791
Residential mortgage-backed securities \| Fixed rate
Investment Holdings [Line Items]
Amortized cost	1,520,414	1,922,618
Unrealized Gains	17,929	0
Unrealized Losses	0	(42,400)
Available-for-sale securities	1,538,343	1,880,218
Residential mortgage-backed securities \| Variable rate
Investment Holdings [Line Items]
Amortized cost		81,270
Unrealized Gains		383
Unrealized Losses		(15)
Available-for-sale securities		$ 81,638
Agency-issued commercial mortgage-backed securities
Investment Holdings [Line Items]
Amortized cost	1,339,651
Unrealized Gains	1,078
Unrealized Losses	(15,539)
Available-for-sale securities	$ 1,325,190

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=118939692&loc=d3e27161-111563

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- Definition

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=118939692&loc=d3e27161-111563

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

+ References

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