Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 13,612,463	$ 14,115,272
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	421,842	524,700
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	5,629,934	7,132,152
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,936,129	1,600,122
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	148,076	178,446
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,383,872	2,812,021
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	3,092,610	$ 1,867,831
After One Year to Five Years, Amortized Cost	134,029
After Five Years to Ten Years, Amortized Cost	540,308
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	13,612,463
Amortized Cost	12,982,514
One Year or Less, Amortized Cost	56,128
One Year or Less, Fair Value	56,991
After One Year to Five Years, Amortized Cost	311,310
After One Year to Five Years, Fair Value	323,717
After Five Years to Ten Years, Amortized Cost	2,031,667
After Five Years to Ten Years, Fair Value	2,142,794
After Ten Years, Amortized Cost	10,583,409
After Ten Years, Fair Value	$ 11,088,961
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	$ 421,842
Amortized Cost	402,346
One Year or Less, Amortized Cost	4,675
One Year or Less, Fair Value	4,720
After One Year to Five Years, Amortized Cost	148,559
After One Year to Five Years, Fair Value	153,787
After Five Years to Ten Years, Amortized Cost	249,112
After Five Years to Ten Years, Fair Value	263,335
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	5,629,934
Amortized Cost	5,363,541
One Year or Less, Amortized Cost	7,113
One Year or Less, Fair Value	7,432
After One Year to Five Years, Amortized Cost	28,722
After One Year to Five Years, Fair Value	29,739
After Five Years to Ten Years, Amortized Cost	588,606
After Five Years to Ten Years, Fair Value	609,151
After Ten Years, Amortized Cost	4,739,100
After Ten Years, Fair Value	4,983,612
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	1,936,129
Amortized Cost	1,909,965
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	551,213
After Five Years to Ten Years, Fair Value	563,127
After Ten Years, Amortized Cost	1,358,752
After Ten Years, Fair Value	1,373,002
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	148,076
Amortized Cost	147,714
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	147,714
After Ten Years, Fair Value	148,076
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,383,872
Amortized Cost	2,229,811
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	102,428
After Five Years to Ten Years, Fair Value	120,085
After Ten Years, Amortized Cost	2,127,383
After Ten Years, Fair Value	2,263,787
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	3,092,610
Amortized Cost	2,929,137
One Year or Less, Amortized Cost	44,340
One Year or Less, Fair Value	44,839
After One Year to Five Years, Fair Value	140,191
After Five Years to Ten Years, Fair Value	587,096
After Ten Years, Amortized Cost	2,210,460
After Ten Years, Fair Value	$ 2,320,484