Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 41,186,735	$ 17,216,871
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	416,189	421,226
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	23,414,311	7,977,673
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,443,238	1,758,382
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	129,037	137,230
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	10,070,499	3,066,805
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	4,662,221	$ 3,855,555
After One Year to Five Years, Amortized Cost	130,482
After Five Years to Ten Years, Amortized Cost	756,078
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	51,240
After One Year to Five Years, Amortized Cost	0
After Five Years to Ten Years, Amortized Cost	51,684
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	41,186,735
Amortized Cost	41,165,732
One Year or Less, Amortized Cost	58,921
One Year or Less, Fair Value	59,890
After One Year to Five Years, Amortized Cost	288,618
After One Year to Five Years, Fair Value	298,861
After Five Years to Ten Years, Amortized Cost	2,546,085
After Five Years to Ten Years, Fair Value	2,624,286
After Ten Years, Amortized Cost	38,272,108
After Ten Years, Fair Value	$ 38,203,698
Held-to-maturity Securities | Lower Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Held-to-maturity Securities | Upper Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
Held-to-maturity Securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 416,189
Amortized Cost	404,571
One Year or Less, Amortized Cost	4,066
One Year or Less, Fair Value	4,117
After One Year to Five Years, Amortized Cost	141,117
After One Year to Five Years, Fair Value	145,321
After Five Years to Ten Years, Amortized Cost	259,388
After Five Years to Ten Years, Fair Value	266,751
After Ten Years, Amortized Cost	0
After Ten Years, Fair Value	0
Held-to-maturity Securities | Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	23,414,311
Amortized Cost	23,525,681
One Year or Less, Amortized Cost	6,319
One Year or Less, Fair Value	6,564
After One Year to Five Years, Amortized Cost	11,663
After One Year to Five Years, Fair Value	11,983
After Five Years to Ten Years, Amortized Cost	494,951
After Five Years to Ten Years, Fair Value	510,743
After Ten Years, Amortized Cost	23,012,748
After Ten Years, Fair Value	22,885,021
Held-to-maturity Securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,443,238
Amortized Cost	2,425,024
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	5,356
After One Year to Five Years, Fair Value	5,482
After Five Years to Ten Years, Amortized Cost	447,781
After Five Years to Ten Years, Fair Value	458,314
After Ten Years, Amortized Cost	1,971,887
After Ten Years, Fair Value	1,979,442
Held-to-maturity Securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	129,037
Amortized Cost	128,370
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	0
After Five Years to Ten Years, Fair Value	0
After Ten Years, Amortized Cost	128,370
After Ten Years, Fair Value	129,037
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	10,070,499
Amortized Cost	10,075,165
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Amortized Cost	536,203
After Five Years to Ten Years, Fair Value	542,183
After Ten Years, Amortized Cost	9,538,962
After Ten Years, Fair Value	9,528,316
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	4,662,221
Amortized Cost	4,555,237
One Year or Less, Amortized Cost	48,536
One Year or Less, Fair Value	49,209
After One Year to Five Years, Fair Value	136,075
After Five Years to Ten Years, Fair Value	795,055
After Ten Years, Amortized Cost	3,620,141
After Ten Years, Fair Value	3,681,882
Held-to-maturity Securities | Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	51,240
Amortized Cost	51,684
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Fair Value	0
After Five Years to Ten Years, Fair Value	51,240
After Ten Years, Amortized Cost	0
After Ten Years, Fair Value	$ 0