Investment Securities - Major Components of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Amortized cost	$ 30,244,896	$ 13,894,348
Unrealized Gains	682,530	140,713
Unrealized Losses	(14,988)	(20,142)
Available-for-sale securities	30,912,438	14,014,919
U.S. treasury securities
Investment Holdings [Line Items]
Amortized cost	4,197,858	6,815,874
Unrealized Gains	271,977	82,267
Unrealized Losses	(107)	(4,131)
Available-for-sale securities	4,469,728	6,894,010
U.S. agency debentures
Investment Holdings [Line Items]
Amortized cost	233,727	100,000
Unrealized Gains	4,165	0
Unrealized Losses	(585)	(453)
Available-for-sale securities	237,307	99,547
Foreign government debt securities
Investment Holdings [Line Items]
Amortized cost	24,491	9,037
Unrealized Gains	1	1
Unrealized Losses	0	0
Available-for-sale securities	24,492	9,038
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Investment Holdings [Line Items]
Amortized cost	13,271,482	4,109,372
Unrealized Gains	232,850	39,438
Unrealized Losses	(651)	(19)
Available-for-sale securities	13,503,681	4,148,791
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations - fixed rate
Investment Holdings [Line Items]
Amortized cost	8,076,832	1,520,414
Unrealized Gains	40,010	17,929
Unrealized Losses	(10,278)	0
Available-for-sale securities	8,106,564	1,538,343
Agency-issued commercial mortgage-backed securities
Investment Holdings [Line Items]
Amortized cost	4,440,506	1,339,651
Unrealized Gains	133,527	1,078
Unrealized Losses	(3,367)	(15,539)
Available-for-sale securities	$ 4,570,666	$ 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=121645371&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=121645371&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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