Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 12,859,045		$ 13,842,946
Unrealized Gains	682,685		289,763
Unrealized Losses	(269)		(17,437)
Fair Value	13,541,461		14,115,272
Allowance for Credit Losses	222	$ 230	0
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	453,280		518,728
Unrealized Gains	20,657		6,640
Unrealized Losses	0		(668)
Fair Value	473,937		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	6,086,154		6,992,009
Unrealized Gains	310,266		142,209
Unrealized Losses	(46)		(2,066)
Fair Value	6,396,374		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	1,518,848		1,608,032
Unrealized Gains	26,124		592
Unrealized Losses	0		(8,502)
Fair Value	1,544,972		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	162,250		178,611
Unrealized Gains	1,403		94
Unrealized Losses	(223)		(259)
Fair Value	163,430		178,446
Allowance for Credit Losses	0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,484,072		2,759,615
Unrealized Gains	162,042		56,914
Unrealized Losses	0		(4,508)
Fair Value	2,646,114		2,812,021
Allowance for Credit Losses	0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,154,441		1,785,951
Unrealized Gains	162,193		83,314
Unrealized Losses	0		(1,434)
Fair Value	2,316,634		1,867,831
Allowance for Credit Losses	$ 222	$ 230	$ 0

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

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