Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 41,165,732	$ 16,592,545
Unrealized Gains	452,837	627,338
Unrealized Losses	(431,834)	(3,012)
Fair Value	41,186,735	17,216,871
Allowance for Credit Losses	1,112	392	$ 230
Available-for-sale securities, transferred to held-to-maturity Securities	2,900,000
Available-for-sale securities, transferred to held-to-maturity securities, unrealized gain (loss)	8,700
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	404,571	402,265
Unrealized Gains	11,618	18,961
Unrealized Losses	0	0
Fair Value	416,189	421,226
Allowance for Credit Losses	0	0
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,525,681	7,739,763
Unrealized Gains	184,691	240,121
Unrealized Losses	(296,061)	(2,211)
Fair Value	23,414,311	7,977,673
Allowance for Credit Losses	0	0
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,425,024	1,735,451
Unrealized Gains	23,807	23,227
Unrealized Losses	(5,593)	(296)
Fair Value	2,443,238	1,758,382
Allowance for Credit Losses	0	0
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	128,370	136,913
Unrealized Gains	667	317
Unrealized Losses	0	0
Fair Value	129,037	137,230
Allowance for Credit Losses	0	0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,075,165	2,942,959
Unrealized Gains	81,135	123,846
Unrealized Losses	(85,801)	0
Fair Value	10,070,499	3,066,805
Allowance for Credit Losses	0	0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,555,237	3,635,194
Unrealized Gains	150,919	220,866
Unrealized Losses	(43,935)	(505)
Fair Value	4,662,221	3,855,555
Allowance for Credit Losses	531	392	$ 230	$ 0
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	51,684	0
Unrealized Gains	0
Unrealized Losses	(444)
Fair Value	51,240
Allowance for Credit Losses	$ 581	$ 0