Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 13,541,461	$ 14,115,272
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	473,937	524,700
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	6,396,374	7,132,152
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,544,972	1,600,122
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	163,430	178,446
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,646,114	2,812,021
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,316,634	$ 1,867,831
After One Year to Five Years, Amortized Cost	142,152
After Five Years to Ten Years, Amortized Cost	479,079
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	13,541,461
Amortized Cost	12,859,045
One Year or Less, Amortized Cost	42,805
One Year or Less, Fair Value	43,227
After One Year to Five Years, Amortized Cost	325,406
After One Year to Five Years, Fair Value	336,222
After Five Years to Ten Years, Amortized Cost	2,142,671
After Five Years to Ten Years, Fair Value	2,251,442
After Ten Years, Amortized Cost	10,348,163
After Ten Years, Fair Value	$ 10,910,570
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	$ 473,937
Amortized Cost	453,280
One Year or Less, Amortized Cost	2,622
One Year or Less, Fair Value	2,666
After One Year to Five Years, Amortized Cost	142,917
After One Year to Five Years, Fair Value	147,089
After Five Years to Ten Years, Amortized Cost	307,741
After Five Years to Ten Years, Fair Value	324,182
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	6,396,374
Amortized Cost	6,086,154
One Year or Less, Amortized Cost	8,369
One Year or Less, Fair Value	8,467
After One Year to Five Years, Amortized Cost	40,337
After One Year to Five Years, Fair Value	41,199
After Five Years to Ten Years, Amortized Cost	640,876
After Five Years to Ten Years, Fair Value	664,186
After Ten Years, Amortized Cost	5,396,572
After Ten Years, Fair Value	5,682,522
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	1,544,972
Amortized Cost	1,518,848
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	612,478
After Five Years to Ten Years, Fair Value	625,694
After Ten Years, Amortized Cost	906,370
After Ten Years, Fair Value	919,278
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	163,430
Amortized Cost	162,250
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	162,250
After Ten Years, Fair Value	163,430
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,646,114
Amortized Cost	2,484,072
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	102,497
After Five Years to Ten Years, Fair Value	120,165
After Ten Years, Amortized Cost	2,381,575
After Ten Years, Fair Value	2,525,949
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,316,634
Amortized Cost	2,154,441
One Year or Less, Amortized Cost	31,814
One Year or Less, Fair Value	32,094
After One Year to Five Years, Fair Value	147,934
After Five Years to Ten Years, Fair Value	517,215
After Ten Years, Amortized Cost	1,501,396
After Ten Years, Fair Value	$ 1,619,391