Investment Securities - Components of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 12,699,542	$ 7,862,311
Unrealized Gains	170,566	12,082
Unrealized Losses	(3,251)	(84,350)
Carrying Value	12,866,857	7,790,043
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,233,957	4,762,182
Unrealized Gains	103,821	11,638
Unrealized Losses	(2,930)	(35,562)
Carrying Value	6,334,848	4,738,258
U.S. agency debentures
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100,000	1,090,426
Unrealized Gains	0	61
Unrealized Losses	0	(6,370)
Carrying Value	100,000	1,084,117
Foreign government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,837	5,815
Unrealized Gains	10	0
Unrealized Losses	0	(3)
Carrying Value	8,847	5,812
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,108,235
Unrealized Gains	40,584
Unrealized Losses	(119)
Carrying Value	4,148,700
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,658,443	1,922,618
Unrealized Gains	21,222	0
Unrealized Losses	(1)	(42,400)
Carrying Value	1,679,664	1,880,218
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—variable rate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		81,270
Unrealized Gains		383
Unrealized Losses		(15)
Carrying Value		$ 81,638
Agency-issued commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	590,070
Unrealized Gains	4,929
Unrealized Losses	(201)
Carrying Value	$ 594,798

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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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.

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