Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 15,188,236	$ 12,548,280
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	638,288	661,545
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,950,882	6,266,295
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,712,301	1,844,116
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,550,756	$ 744,213
After One Year to Five Years - Amortized Cost	75,379
After Five Years to Ten Years - Amortized Cost	307,184
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,487,442
Fair Value	15,188,236
One Year or Less - Amortized Cost	9,725
One Year or Less - Fair Value	9,720
After One Year to Five Years - Amortized Cost	335,186
After One Year to Five Years - Fair Value	331,299
After Five Years to Ten Years - Amortized Cost	2,212,289
After Five Years to Ten Years - Fair Value	2,161,016
After Ten Years - Amortized Cost	12,930,242
After Ten Years - Fair Value	12,686,201
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	640,990
Fair Value	638,288
One Year or Less - Amortized Cost	0
One Year or Less - Fair Value	0
After One Year to Five Years - Amortized Cost	104,550
After One Year to Five Years - Fair Value	104,105
After Five Years to Ten Years - Amortized Cost	536,440
After Five Years to Ten Years - Fair Value	534,183
After Ten Years - Amortized Cost	0
After Ten Years - Fair Value	0
Held-to-maturity securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,103,638
Fair Value	7,950,883
One Year or Less - Amortized Cost	0
One Year or Less - Fair Value	0
After One Year to Five Years - Amortized Cost	155,257
After One Year to Five Years - Fair Value	152,524
After Five Years to Ten Years - Amortized Cost	885,622
After Five Years to Ten Years - Fair Value	859,609
After Ten Years - Amortized Cost	7,062,759
After Ten Years - Fair Value	6,938,750
Held-to-maturity securities | Residential mortgage-backed securities | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,183,204
Fair Value	2,120,932
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	483,043
After Five Years to Ten Years - Fair Value	466,458
After Ten Years - Amortized Cost	1,700,161
After Ten Years - Fair Value	1,654,474
Held-to-maturity securities | Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	214,483
Fair Value	215,077
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	214,483
After Ten Years - Fair Value	215,077
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,769,706
Fair Value	2,712,301
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	2,769,706
After Ten Years - Fair Value	2,712,301
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,575,421
Fair Value	1,550,755
One Year or Less - Amortized Cost	9,725
One Year or Less - Fair Value	9,720
After One Year to Five Years - Fair Value	74,670
After Five Years to Ten Years - Fair Value	300,766
After Ten Years - Amortized Cost	1,183,133
After Ten Years - Fair Value	$ 1,165,599