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, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 7,735,891	$ 7,421,042
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 7,735,891
Weighted- Average Yield	2.07%
One Year or Less - Amortized Cost	$ 3,544
One Year or Less - Weighted- Average Yield	5.50%
One Year to Five Years - Amortized Cost	$ 70,669
After One Year to Five Years - Weighted- Average Yield	3.87%
After Five Years to Ten Years - Amortized Cost	$ 1,258,699
After Five Years to Ten Years - Weighted- Average Yield	2.50%
After Ten Years - Amortized Cost	$ 6,402,979
After Ten Years - Weighted- Average Yield	1.96%
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 488,185
Weighted- Average Yield	2.68%
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	$ 488,185
After Five Years to Ten Years - Weighted- Average Yield	2.68%
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	$ 2,590,151
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	$ 40,936
After One Year to Five Years - Weighted- Average Yield	2.38%
After Five Years to Ten Years - Amortized Cost	$ 733,723
After Five Years to Ten Years - Weighted- Average Yield	2.21%
After Ten Years - Amortized Cost	$ 1,815,492
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	$ 3,527,554
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	$ 3,527,554
After Ten Years - Weighted- Average Yield	1.65%
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	$ 117,109
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	$ 117,109
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	$ 936,337
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	$ 936,337
After Ten Years - Weighted- Average Yield	2.16%
Held-to-maturity securities | Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 76,555
Weighted- Average Yield	6.04%
One Year or Less - Amortized Cost	$ 3,544
One Year or Less - Weighted- Average Yield	5.50%
One Year to Five Years - Amortized Cost	$ 29,733
After One Year to Five Years - Weighted- Average Yield	5.92%
After Five Years to Ten Years - Amortized Cost	$ 36,791
After Five Years to Ten Years - Weighted- Average Yield	6.14%
After Ten Years - Amortized Cost	$ 6,487
After Ten Years - Weighted- Average Yield	6.34%
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