Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 15,064,962	$ 15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	599,928	638,288
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,803,367	7,950,882
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,900,297	2,120,932
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	197,714	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,923,560	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,640,096	$ 1,550,756
After One Year to Five Years, Amortized Cost	83,468
After Five Years to Ten Years, Amortized Cost	340,526
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	15,064,962
Amortized Cost	14,868,761
One Year or Less, Amortized Cost	14,977
One Year or Less, Fair Value	14,984
After One Year to Five Years, Amortized Cost	295,565
After One Year to Five Years, Fair Value	298,661
After Five Years to Ten Years, Amortized Cost	2,152,728
After Five Years to Ten Years, Fair Value	2,166,758
After Ten Years, Amortized Cost	12,405,491
After Ten Years, Fair Value	$ 12,584,559
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	$ 599,928
Amortized Cost	585,817
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	103,530
After Five Years to Ten Years, Amortized Cost	484,353
After Five Years to Ten Years, Fair Value	496,398
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	7,803,367
Amortized Cost	7,706,133
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	110,633
After One Year to Five Years, Fair Value	110,834
After Five Years to Ten Years, Amortized Cost	821,076
After Five Years to Ten Years, Fair Value	817,428
After Ten Years, Amortized Cost	6,774,424
After Ten Years, Fair Value	6,875,105
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	1,900,297
Amortized Cost	1,912,839
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	506,773
After Five Years to Ten Years, Fair Value	501,451
After Ten Years, Amortized Cost	1,406,066
After Ten Years, Fair Value	1,398,846
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	197,714
Amortized Cost	197,962
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	197,962
After Ten Years, Fair Value	197,714
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,923,560
Amortized Cost	2,886,958
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	2,886,958
After Ten Years, Fair Value	2,923,560
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,640,096
Amortized Cost	1,579,052
One Year or Less, Amortized Cost	14,977
One Year or Less, Fair Value	14,984
After One Year to Five Years, Fair Value	84,297
After Five Years to Ten Years, Fair Value	351,481
After Ten Years, Amortized Cost	1,140,081
After Ten Years, Fair Value	$ 1,189,334