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, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 8,306,526	$ 7,421,042
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,306,526
Weighted- Average Yield	2.06%
One Year or Less - Amortized Cost	$ 4,711
One Year or Less - Weighted- Average Yield	5.58%
One Year to Five Years - Amortized Cost	$ 68,610
After One Year to Five Years - Weighted- Average Yield	3.89%
After Five Years to Ten Years - Amortized Cost	$ 1,256,035
After Five Years to Ten Years - Weighted- Average Yield	2.53%
After Ten Years - Amortized Cost	$ 6,977,170
After Ten Years - Weighted- Average Yield	1.95%
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 545,753
Weighted- Average Yield	2.69%
One Year or Less - Amortized Cost
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	$ 545,753
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%
Held-to-maturity securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,457,328
Weighted- Average Yield	2.43%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
One Year to Five Years - Amortized Cost	$ 39,527
After One Year to Five Years - Weighted- Average Yield	2.38%
After Five Years to Ten Years - Amortized Cost	$ 674,584
After Five Years to Ten Years - Weighted- Average Yield	2.20%
After Ten Years - Amortized Cost	$ 1,743,217
After Ten Years - Weighted- Average Yield	2.52%
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	$ 4,045,172
Weighted- Average Yield	1.70%
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	$ 4,045,172
After Ten Years - Weighted- Average Yield	1.70%
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	$ 109,478
Weighted- Average Yield	0.65%
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	$ 109,478
After Ten Years - Weighted- Average Yield	0.65%
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,073,160
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	$ 1,073,160
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	$ 75,635
Weighted- Average Yield	6.05%
One Year or Less - Amortized Cost	$ 4,711
One Year or Less - Weighted- Average Yield	5.58%
One Year to Five Years - Amortized Cost	$ 29,083
After One Year to Five Years - Weighted- Average Yield	5.95%
After Five Years to Ten Years - Amortized Cost	$ 35,698
After Five Years to Ten Years - Weighted- Average Yield	6.14%
After Ten Years - Amortized Cost	$ 6,143
After Ten Years - Weighted- Average Yield	6.35%
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