Investment Securities - Summary of Remaining Contractual Principal Maturities for Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	$ 14,698,802	$ 15,188,236
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	529,347	638,288
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	7,508,264	7,950,882
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,746,658	2,120,932
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	187,852	215,077
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,898,720	2,712,301
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,827,961	$ 1,550,756
After One Year to Five Years, Amortized Cost	82,708
After Five Years to Ten Years, Amortized Cost	369,911
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	14,698,802
Amortized Cost	14,407,078
One Year or Less, Amortized Cost	14,002
One Year or Less, Fair Value	14,007
After One Year to Five Years, Amortized Cost	300,427
After One Year to Five Years, Fair Value	303,105
After Five Years to Ten Years, Amortized Cost	2,172,452
After Five Years to Ten Years, Fair Value	2,190,040
After Ten Years, Amortized Cost	11,920,197
After Ten Years, Fair Value	$ 12,191,650
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	$ 529,347
Amortized Cost	518,841
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	123,205
After One Year to Five Years, Fair Value	124,646
After Five Years to Ten Years, Amortized Cost	395,636
After Five Years to Ten Years, Fair Value	404,701
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	7,508,264
Amortized Cost	7,376,458
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year to Five Years, Amortized Cost	94,514
After One Year to Five Years, Fair Value	94,745
After Five Years to Ten Years, Amortized Cost	771,670
After Five Years to Ten Years, Fair Value	769,409
After Ten Years, Amortized Cost	6,510,274
After Ten Years, Fair Value	6,644,110
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	1,746,658
Amortized Cost	1,754,498
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	635,235
After Five Years to Ten Years, Fair Value	630,472
After Ten Years, Amortized Cost	1,119,263
After Ten Years, Fair Value	1,116,186
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	187,852
Amortized Cost	188,120
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	188,120
After Ten Years, Fair Value	187,852
Held-to-maturity Securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	2,898,720
Amortized Cost	2,826,344
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	2,826,344
After Ten Years, Fair Value	2,898,720
Held-to-maturity Securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Fair Value	1,827,961
Amortized Cost	1,742,817
One Year or Less, Amortized Cost	14,002
One Year or Less, Fair Value	14,007
After One Year to Five Years, Fair Value	83,714
After Five Years to Ten Years, Fair Value	385,458
After Ten Years, Amortized Cost	1,276,196
After Ten Years, Fair Value	$ 1,344,782