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	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 8,200,443	$ 8,790,963
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,200,443
Weighted- Average Yield	2.02%
One Year or Less - Amortized Cost	$ 4,627
One Year or Less - Weighted- Average Yield	5.69%
After One Year to Five Years - Amortized Cost	$ 266,164
After One Year to Five Years - Weighted- Average Yield	3.12%
After Five Years to Ten Years - Amortized Cost	$ 921,139
After Five Years to Ten Years - Weighted- Average Yield	2.45%
After Ten Years - Amortized Cost	$ 7,008,513
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	$ 588,813
Weighted- Average Yield	2.70%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
After One Year to Five Years - Amortized Cost	$ 16,633
After One Year to Five Years - Weighted- Average Yield	4.07%
After Five Years to Ten Years - Amortized Cost	$ 572,180
After Five Years to Ten Years - Weighted- Average Yield	2.66%
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	$ 2,228,633
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	$ 223,864
After One Year to Five Years - Weighted- Average Yield	2.72%
After Five Years to Ten Years - Amortized Cost	$ 299,976
After Five Years to Ten Years - Weighted- Average Yield	1.81%
After Ten Years - Amortized Cost	$ 1,704,793
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	$ 3,829,568
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	$ 23,606
After Five Years to Ten Years - Weighted- Average Yield	1.75%
After Ten Years - Amortized Cost	$ 3,805,962
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	$ 343,040
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	$ 343,040
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	$ 1,152,105
Weighted- Average Yield	2.12%
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	$ 1,152,105
After Ten Years - Weighted- Average Yield	2.12%
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 58,284
Weighted- Average Yield	6.06%
One Year or Less - Amortized Cost	$ 4,627
One Year or Less - Weighted- Average Yield	5.69%
After One Year to Five Years - Amortized Cost	$ 25,667
After One Year to Five Years - Weighted- Average Yield	6.01%
After Five Years to Ten Years - Amortized Cost	$ 25,377
After Five Years to Ten Years - Weighted- Average Yield	6.14%
After Ten Years - Amortized Cost	$ 2,613
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