Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 12,982,514		$ 13,842,946
Unrealized Gains	638,566		289,763
Unrealized Losses	(8,617)		(17,437)
Fair Value	13,612,463		14,115,272
Allowance for Credit Losses	291	$ 222	0
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	402,346		518,728
Unrealized Gains	19,496		6,640
Unrealized Losses	0		(668)
Fair Value	421,842		524,700
Allowance for Credit Losses	0
Residential mortgage-backed securities \| Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,363,541		6,992,009
Unrealized Gains	266,455		142,209
Unrealized Losses	(62)		(2,066)
Fair Value	5,629,934		7,132,152
Allowance for Credit Losses	0
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,909,965		1,608,032
Unrealized Gains	26,601		592
Unrealized Losses	(437)		(8,502)
Fair Value	1,936,129		1,600,122
Allowance for Credit Losses	0
Residential mortgage-backed securities \| Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	147,714		178,611
Unrealized Gains	362		94
Unrealized Losses	0		(259)
Fair Value	148,076		178,446
Allowance for Credit Losses	0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,229,811		2,759,615
Unrealized Gains	154,061		56,914
Unrealized Losses	0		(4,508)
Fair Value	2,383,872		2,812,021
Allowance for Credit Losses	0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,929,137		1,785,951
Unrealized Gains	171,591		83,314
Unrealized Losses	(8,118)		(1,434)
Fair Value	3,092,610		1,867,831
Allowance for Credit Losses	$ 291	$ 222	$ 0

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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