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	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 7,816,797	$ 7,421,042
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,816,797
Weighted- Average Yield	2.07%
One Year or Less - Amortized Cost	3,442
One Year or Less - Weighted- Average Yield	5.39%
One Year to Five Years - Amortized Cost	75,643
After One Year to Five Years - Weighted- Average Yield	3.91%
After Five Years to Ten Years - Amortized Cost	1,308,170
After Five Years to Ten Years - Weighted- Average Yield	2.52%
After Ten Years - Amortized Cost	6,429,542
After Ten Years - Weighted- Average Yield	1.95%
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	473,373
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	473,373
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	2,693,488
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	42,382
After One Year to Five Years - Weighted- Average Yield	2.38%
After Five Years to Ten Years - Amortized Cost	794,716
After Five Years to Ten Years - Weighted- Average Yield	2.23%
After Ten Years - Amortized Cost	1,856,390
After Ten Years - Weighted- Average Yield	2.51%
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	3,553,640
Weighted- Average Yield	1.65%
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	3,553,640
After Ten Years - Weighted- Average Yield	1.65%
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	124,195
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	124,195
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	888,823
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	888,823
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	83,278
Weighted- Average Yield	6.00%
One Year or Less - Amortized Cost	3,442
One Year or Less - Weighted- Average Yield	5.39%
One Year to Five Years - Amortized Cost	33,261
After One Year to Five Years - Weighted- Average Yield	5.87%
After Five Years to Ten Years - Amortized Cost	40,081
After Five Years to Ten Years - Weighted- Average Yield	6.11%
After Ten Years - Amortized Cost	$ 6,494
After Ten Years - Weighted- Average Yield	6.34%