Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 17,800,589	$ 13,894,348
Unrealized Gains	655,900	140,713
Unrealized Losses	(4,576)	(20,142)
Carrying Value	18,451,913	14,014,919
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,229,988	6,815,874
Unrealized Gains	305,252	82,267
Unrealized Losses	(5)	(4,131)
Carrying Value	4,535,235	6,894,010
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100,000	100,000
Unrealized Gains	2,659	0
Unrealized Losses	0	(453)
Carrying Value	102,659	99,547
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,540	9,037
Unrealized Gains	0	1
Unrealized Losses	(15)	0
Carrying Value	22,525	9,038
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,465,130	4,109,372
Unrealized Gains	193,484	39,438
Unrealized Losses	(3,058)	(19)
Carrying Value	7,655,556	4,148,791
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,949,805	1,520,414
Unrealized Gains	31,229	17,929
Unrealized Losses	(1,494)	0
Carrying Value	2,979,540	1,538,343
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,033,126	1,339,651
Unrealized Gains	123,276	1,078
Unrealized Losses	(4)	(15,539)
Carrying Value	$ 3,156,398	$ 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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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.

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