Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,039,762
Available-for-sale Securities, Due after one year through five years, Amortized Cost	151,635
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	512,451
Available-for-sale Securities, Due after ten years, Amortized Cost	759,832
Available-for-sale Securities, Amortized Cost Basis	2,463,680	$ 2,779,047
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,040,780
Available-for-sale Securities, Due after one year through five years, Fair Value	151,911
Available-for-sale Securities, Due after five years through ten years, Fair Value	511,166
Available-for-sale Securities, Due after ten years, Fair Value	756,822
Available-for-sale securities	$ 2,460,679	$ 2,775,025
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.57%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.19%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.03%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.10%
Available-for-sale Securities, Total Weighted-Average Yield	0.89%