Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Held to Maturity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 7,791,949	$ 8,790,963
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 7,791,949
Weighted- Average Yield	2.01%
One Year or Less - Amortized Cost	$ 4,076
One Year or Less - Weighted- Average Yield	5.76%
After One Year to Five Years - Amortized Cost	$ 329,169
After One Year to Five Years - Weighted- Average Yield	2.93%
After Five Years to Ten Years - Amortized Cost	$ 837,848
After Five Years to Ten Years - Weighted- Average Yield	2.29%
After Ten Years - Amortized Cost	$ 6,620,856
After Ten Years - Weighted- Average Yield	1.92%
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 622,692
Weighted- Average Yield	2.43%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 47,478
After One Year to Five Years - Weighted- Average Yield	3.25%
After Five Years to Ten Years - Amortized Cost	$ 575,214
After Five Years to Ten Years - Weighted- Average Yield	2.36%
After Ten Years - Amortized Cost	$ 0
After Ten Years - Weighted- Average Yield	0.00%
Held-to-maturity securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,092,082
Weighted- Average Yield	2.38%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 257,766
After One Year to Five Years - Weighted- Average Yield	2.58%
After Five Years to Ten Years - Amortized Cost	$ 216,197
After Five Years to Ten Years - Weighted- Average Yield	1.74%
After Ten Years - Amortized Cost	$ 1,618,119
After Ten Years - Weighted- Average Yield	2.44%
Held-to-maturity securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,593,453
Weighted- Average Yield	1.73%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Amortized Cost	$ 22,233
After Five Years to Ten Years - Weighted- Average Yield	1.75%
After Ten Years - Amortized Cost	$ 3,571,220
After Ten Years - Weighted- Average Yield	1.73%
Held-to-maturity securities | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 327,673
Weighted- Average Yield	0.74%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Amortized Cost	$ 0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Amortized Cost	$ 327,673
After Ten Years - Weighted- Average Yield	0.74%
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,101,234
Weighted- Average Yield	2.13%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 0
After One Year to Five Years - Weighted- Average Yield	0.00%
After Five Years to Ten Years - Amortized Cost	$ 0
After Five Years to Ten Years - Weighted- Average Yield	0.00%
After Ten Years - Amortized Cost	$ 1,101,234
After Ten Years - Weighted- Average Yield	2.13%
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 54,815
Weighted- Average Yield	6.07%
One Year or Less - Amortized Cost	$ 4,076
One Year or Less - Weighted- Average Yield	5.76%
After One Year to Five Years - Amortized Cost	$ 23,925
After One Year to Five Years - Weighted- Average Yield	6.02%
After Five Years to Ten Years - Amortized Cost	$ 24,204
After Five Years to Ten Years - Weighted- Average Yield	6.13%
After Ten Years - Amortized Cost	$ 2,610
After Ten Years - Weighted- Average Yield	6.46%
Lower Limit | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Upper Limit | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years