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	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%	35.00%	35.00%
Amortized Cost	$ 8,790,963	$ 7,421,042
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,790,963
Weighted- Average Yield	2.01%
One Year or Less - Amortized Cost	$ 4,674
One Year or Less - Weighted- Average Yield	5.58%
One Year to Five Years - Amortized Cost	$ 66,329
After One Year to Five Years - Weighted- Average Yield	3.88%
After Five Years to Ten Years - Amortized Cost	$ 1,197,752
After Five Years to Ten Years - Weighted- Average Yield	2.52%
After Ten Years - Amortized Cost	$ 7,522,208
After Ten Years - Weighted- Average Yield	1.91%
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
U.S. agency debentures | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 545,473
Weighted- Average Yield	2.69%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
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	$ 545,473
After Five Years to Ten Years - Weighted- Average Yield	2.69%
After Ten Years - Amortized Cost	$ 0
After Ten Years - Weighted- Average Yield	0.00%
Agency-issued mortgage-backed securities | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,366,627
Weighted- Average Yield	2.40%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
One Year to Five Years - Amortized Cost	$ 38,436
After One Year to Five Years - Weighted- Average Yield	2.38%
After Five Years to Ten Years - Amortized Cost	$ 621,748
After Five Years to Ten Years - Weighted- Average Yield	2.20%
After Ten Years - Amortized Cost	$ 1,706,443
After Ten Years - Weighted- Average Yield	2.47%
Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 4,225,781
Weighted- Average Yield	1.72%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
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	$ 4,225,781
After Ten Years - Weighted- Average Yield	1.72%
Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 370,779
Weighted- Average Yield	0.74%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
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	$ 370,779
After Ten Years - Weighted- Average Yield	0.74%
Agency-issued commercial mortgage-backed securities | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,214,716
Weighted- Average Yield	2.12%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
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	$ 1,214,716
After Ten Years - Weighted- Average Yield	2.12%
Municipal bonds and notes | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 67,587
Weighted- Average Yield	6.04%
One Year or Less - Amortized Cost	$ 4,674
One Year or Less - Weighted- Average Yield	5.58%
One Year to Five Years - Amortized Cost	$ 27,893
After One Year to Five Years - Weighted- Average Yield	5.95%
After Five Years to Ten Years - Amortized Cost	$ 30,531
After Five Years to Ten Years - Weighted- Average Yield	6.14%
After Ten Years - Amortized Cost	$ 4,489
After Ten Years - Weighted- Average Yield	6.34%