Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 17,216,871	$ 14,115,272
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	421,226	524,700
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,977,673	7,132,152
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations - fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,758,382	1,600,122
Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	137,230	178,446
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	3,066,805	2,812,021
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	3,855,555	$ 1,867,831
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	16,592,545
Fair Value	17,216,871
One Year or Less - Amortized Cost	55,729
One Year or Less - Fair Value	56,297
After One Year to Five Years - Amortized Cost	319,166
After One Year to Five Years - Fair Value	331,919
After Five Years to Ten Years - Amortized Cost	2,056,015
After Five Years to Ten Years - Fair Value	2,169,124
After Ten Years - Amortized Cost	14,161,635
After Ten Years - Fair Value	14,659,531
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	402,265
Fair Value	421,226
One Year or Less - Amortized Cost	4,675
One Year or Less - Fair Value	4,705
After One Year to Five Years - Amortized Cost	148,478
After One Year to Five Years - Fair Value	153,756
After Five Years to Ten Years - Amortized Cost	249,112
After Five Years to Ten Years - Fair Value	262,765
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]
Amortized Cost	7,739,763
Fair Value	7,977,673
One Year or Less - Amortized Cost	4,762
One Year or Less - Fair Value	4,951
After One Year to Five Years - Amortized Cost	20,389
After One Year to Five Years - Fair Value	21,150
After Five Years to Ten Years - Amortized Cost	540,731
After Five Years to Ten Years - Fair Value	559,727
After Ten Years - Amortized Cost	7,173,881
After Ten Years - Fair Value	7,391,845
Held-to-maturity securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations - fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,735,451
Fair Value	1,758,382
One Year or Less - Amortized Cost	0
One Year or Less - Fair Value	0
After One Year to Five Years - Amortized Cost	5,952
After One Year to Five Years - Fair Value	6,073
After Five Years to Ten Years - Amortized Cost	494,532
After Five Years to Ten Years - Fair Value	505,156
After Ten Years - Amortized Cost	1,234,967
After Ten Years - Fair Value	1,247,153
Held-to-maturity securities | Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	136,913
Fair Value	137,230
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	136,913
After Ten Years - Fair Value	137,230
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,942,959
Fair Value	3,066,805
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	102,359
After Five Years to Ten Years - Fair Value	119,922
After Ten Years - Amortized Cost	2,840,600
After Ten Years - Fair Value	2,946,883
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,635,194
Fair Value	3,855,555
One Year or Less - Amortized Cost	46,292
One Year or Less - Fair Value	46,641
After One Year to Five Years - Amortized Cost	144,347
After One Year to Five Years - Fair Value	150,940
After Five Years to Ten Years - Amortized Cost	669,281
After Five Years to Ten Years - Fair Value	721,554
After Ten Years - Amortized Cost	2,775,274
After Ten Years - Fair Value	$ 2,936,420