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	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Federal statutory tax rate	35.00%
Amortized Cost	$ 8,548,238	$ 8,790,963
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,548,238
Weighted- Average Yield	2.03%
One Year or Less - Amortized Cost	$ 5,192
One Year or Less - Weighted- Average Yield	5.59%
One Year to Five Years - Amortized Cost	$ 268,360
After One Year to Five Years - Weighted- Average Yield	3.20%
After Five Years to Ten Years - Amortized Cost	$ 974,230
After Five Years to Ten Years - Weighted- Average Yield	2.43%
After Ten Years - Amortized Cost	$ 7,300,456
After Ten Years - Weighted- Average Yield	1.93%
Held-to-maturity securities | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 589,076
Weighted- Average Yield	2.70%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
One Year to Five Years - Amortized Cost	$ 16,702
After One Year to Five Years - Weighted- Average Yield	4.07%
After Five Years to Ten Years - Amortized Cost	$ 572,374
After Five Years to Ten Years - Weighted- Average Yield	2.66%
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	$ 2,308,454
Weighted- Average Yield	2.44%
One Year or Less - Amortized Cost	$ 0
One Year or Less - Weighted- Average Yield	0.00%
One Year to Five Years - Amortized Cost	$ 222,948
After One Year to Five Years - Weighted- Average Yield	2.78%
After Five Years to Ten Years - Amortized Cost	$ 350,836
After Five Years to Ten Years - Weighted- Average Yield	1.84%
After Ten Years - Amortized Cost	$ 1,734,670
After Ten Years - Weighted- Average Yield	2.51%
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	$ 4,047,342
Weighted- Average Yield	1.73%
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	$ 25,320
After Five Years to Ten Years - Weighted- Average Yield	1.75%
After Ten Years - Amortized Cost	$ 4,022,022
After Ten Years - Weighted- Average Yield	1.73%
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	$ 359,244
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	$ 359,244
After Ten Years - Weighted- Average Yield	0.74%
Held-to-maturity securities | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,181,904
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	$ 1,181,904
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	$ 62,218
Weighted- Average Yield	6.03%
One Year or Less - Amortized Cost	$ 5,192
One Year or Less - Weighted- Average Yield	5.59%
One Year to Five Years - Amortized Cost	$ 28,710
After One Year to Five Years - Weighted- Average Yield	5.98%
After Five Years to Ten Years - Amortized Cost	$ 25,700
After Five Years to Ten Years - Weighted- Average Yield	6.14%
After Ten Years - Amortized Cost	$ 2,616
After Ten Years - Weighted- Average Yield	6.46%
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