Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 14,115,272	$ 15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	524,700	638,288
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,132,152	7,950,882
Residential mortgage-backed securities | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,600,122	2,120,932
Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	178,446	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,812,021	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,867,831	$ 1,550,756
After One Year to Five Years - Amortized Cost	83,368
After Five Years to Ten Years - Amortized Cost	387,278
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	13,842,946
Fair Value	14,115,272
One Year or Less - Amortized Cost	16,039
One Year or Less - Fair Value	16,101
After One Year to Five Years - Amortized Cost	283,227
After One Year to Five Years - Fair Value	285,698
After Five Years to Ten Years - Amortized Cost	2,236,089
After Five Years to Ten Years - Fair Value	2,260,337
After Ten Years - Amortized Cost	11,307,591
After Ten Years - Fair Value	11,553,136
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	518,728
Fair Value	524,700
One Year or Less - Amortized Cost	0
One Year or Less - Fair Value	0
After One Year to Five Years - Amortized Cost	123,100
After One Year to Five Years - Fair Value	123,969
After Five Years to Ten Years - Amortized Cost	395,628
After Five Years to Ten Years - Fair Value	400,731
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	6,992,009
Fair Value	7,132,152
One Year or Less - Amortized Cost	2,066
One Year or Less - Fair Value	2,117
After One Year to Five Years - Amortized Cost	76,759
After One Year to Five Years - Fair Value	76,956
After Five Years to Ten Years - Amortized Cost	726,422
After Five Years to Ten Years - Fair Value	725,854
After Ten Years - Amortized Cost	6,186,762
After Ten Years - Fair Value	6,327,225
Held-to-maturity securities | Residential mortgage-backed securities | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,608,032
Fair Value	1,600,122
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	624,128
After Five Years to Ten Years - Fair Value	619,180
After Ten Years - Amortized Cost	983,904
After Ten Years - Fair Value	980,942
Held-to-maturity securities | Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	178,611
Fair Value	178,446
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	178,611
After Ten Years - Fair Value	178,446
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,759,615
Fair Value	2,812,021
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	102,633
After Five Years to Ten Years - Fair Value	110,836
After Ten Years - Amortized Cost	2,656,982
After Ten Years - Fair Value	2,701,185
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,785,951
Fair Value	1,867,831
One Year or Less - Amortized Cost	13,973
One Year or Less - Fair Value	13,984
After One Year to Five Years - Fair Value	84,773
After Five Years to Ten Years - Fair Value	403,736
After Ten Years - Amortized Cost	1,301,332
After Ten Years - Fair Value	$ 1,365,338