Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 13,574,519	$ 13,842,946
Unrealized Gains	560,684	289,763
Unrealized Losses	(4,049)	(17,437)
Fair Value	14,131,154	14,115,272
Allowance for Credit Losses	230	0
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	453,353	518,728
Unrealized Gains	14,799	6,640
Unrealized Losses	0	(668)
Fair Value	468,152	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	6,662,878	6,992,009
Unrealized Gains	310,896	142,209
Unrealized Losses	(90)	(2,066)
Fair Value	6,973,684	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	1,482,205	1,608,032
Unrealized Gains	30,323	592
Unrealized Losses	(4)	(8,502)
Fair Value	1,512,524	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	170,774	178,611
Unrealized Gains	0	94
Unrealized Losses	(1,652)	(259)
Fair Value	169,122	178,446
Allowance for Credit Losses	0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,644,968	2,759,615
Unrealized Gains	98,806	56,914
Unrealized Losses	(512)	(4,508)
Fair Value	2,743,262	2,812,021
Allowance for Credit Losses	0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,160,341	1,785,951
Unrealized Gains	105,860	83,314
Unrealized Losses	(1,791)	(1,434)
Fair Value	2,264,410	1,867,831
Allowance for Credit Losses	$ 230	$ 0