Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 14,996,508	$ 15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	585,800	638,288
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,877,941	7,950,882
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,708,852	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,599,666	$ 1,550,756
After One Year to Five Years, Amortized Cost	80,710
After Five Years to Ten Years, Amortized Cost	318,981
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	14,996,508
Amortized Cost	15,055,255
One Year or Less, Amortized Cost	13,230
One Year or Less, Fair Value	13,231
After One Year to Five Years, Amortized Cost	315,195
After One Year to Five Years, Fair Value	314,613
After Five Years to Ten Years, Amortized Cost	2,186,592
After Five Years to Ten Years, Fair Value	2,163,417
After Ten Years, Amortized Cost	12,540,238
After Ten Years, Fair Value	$ 12,505,247
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	$ 585,800
Amortized Cost	585,892
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	101,464
After One Year to Five Years, Fair Value	101,854
After Five Years to Ten Years, Amortized Cost	484,428
After Five Years to Ten Years, Fair Value	483,946
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]
Fair Value	7,877,941
Amortized Cost	7,887,579
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	133,021
After One Year to Five Years, Fair Value	131,935
After Five Years to Ten Years, Amortized Cost	868,170
After Five Years to Ten Years, Fair Value	854,690
After Ten Years, Amortized Cost	6,886,388
After Ten Years, Fair Value	6,891,316
Held-to-maturity Securities | Residential mortgage-backed securities | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,018,441
Amortized Cost	2,058,789
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	515,013
After Five Years to Ten Years, Fair Value	501,867
After Ten Years, Amortized Cost	1,543,776
After Ten Years, Fair Value	1,516,574
Held-to-maturity Securities | Residential mortgage-backed securities | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	205,808
Amortized Cost	206,421
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	206,421
After Ten Years, Fair Value	205,808
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,708,852
Amortized Cost	2,745,662
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	2,745,662
After Ten Years, Fair Value	2,708,852
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,599,666
Amortized Cost	1,570,912
One Year or Less, Amortized Cost	13,230
One Year or Less, Fair Value	13,231
After One Year to Five Years, Fair Value	80,824
After Five Years to Ten Years, Fair Value	322,914
After Ten Years, Amortized Cost	1,157,991
After Ten Years, Fair Value	$ 1,182,697