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, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 6,662,025	$ 0
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,662,025
Weighted- Average Yield	2.15%
One Year or Less - Amortized Cost	3,470
One Year or Less - Weighted- Average Yield	5.37%
One Year to Five Years - Amortized Cost	72,981
After One Year to Five Years - Weighted- Average Yield	3.71%
After Five Years to Ten Years - Amortized Cost	1,292,189
After Five Years to Ten Years - Weighted- Average Yield	2.51%
After Ten Years - Amortized Cost	5,293,385
After Ten Years - Weighted- Average Yield	2.03%
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
Agency-issued mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,919,715
Weighted- Average Yield	2.42%
One Year or Less - Amortized Cost	0
One Year or Less - Weighted- Average Yield	0.00%
One Year to Five Years - Amortized Cost	45,081
After One Year to Five Years - Weighted- Average Yield	2.39%
After Five Years to Ten Years - Amortized Cost	901,721
After Five Years to Ten Years - Weighted- Average Yield	2.20%
After Ten Years - Amortized Cost	1,972,913
After Ten Years - Weighted- Average Yield	2.52%
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	2,489,538
Weighted- Average Yield	1.68%
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	2,489,538
After Ten Years - Weighted- Average Yield	1.68%
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	139,182
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	139,182
After Ten Years - Weighted- Average Yield	0.65%
Agency-issued commercial mortgage-backed securities | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	679,379
Weighted- Average Yield	2.16%
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	679,379
After Ten Years - Weighted- Average Yield	2.16%
Municipal bonds and notes | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	84,218
Weighted- Average Yield	6.00%
One Year or Less - Amortized Cost	3,470
One Year or Less - Weighted- Average Yield	5.37%
One Year to Five Years - Amortized Cost	27,900
After One Year to Five Years - Weighted- Average Yield	5.84%
After Five Years to Ten Years - Amortized Cost	40,475
After Five Years to Ten Years - Weighted- Average Yield	6.07%
After Ten Years - Amortized Cost	12,373
After Ten Years - Weighted- Average Yield	6.30%
U.S. agency debentures | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	349,993
Weighted- Average Yield	2.90%
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	349,993
After Five Years to Ten Years - Weighted- Average Yield	2.90%
After Ten Years - Amortized Cost	$ 0
After Ten Years - Weighted- Average Yield	0.00%