Investment Securities - Summary of Components and Unrealized Losses on Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 16,592,545	$ 13,842,946
Unrealized Gains	627,338	289,763
Unrealized Losses	(3,012)	(17,437)
Fair Value	17,216,871	14,115,272
Allowance for Credit Losses	392	0
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	402,265	518,728
Unrealized Gains	18,961	6,640
Unrealized Losses	0	(668)
Fair Value	421,226	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	7,739,763	6,992,009
Unrealized Gains	240,121	142,209
Unrealized Losses	(2,211)	(2,066)
Fair Value	7,977,673	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	1,735,451	1,608,032
Unrealized Gains	23,227	592
Unrealized Losses	(296)	(8,502)
Fair Value	1,758,382	1,600,122
Allowance for Credit Losses	0
Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	136,913	178,611
Unrealized Gains	317	94
Unrealized Losses	0	(259)
Fair Value	137,230	178,446
Allowance for Credit Losses	0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	2,942,959	2,759,615
Unrealized Gains	123,846	56,914
Unrealized Losses	0	(4,508)
Fair Value	3,066,805	2,812,021
Allowance for Credit Losses	0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	3,635,194	1,785,951
Unrealized Gains	220,866	83,314
Unrealized Losses	(505)	(1,434)
Fair Value	3,855,555	1,867,831
Allowance for Credit Losses	$ 392	$ 0