Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 925,228
Available-for-sale Securities, Due after one year through five years, Amortized Cost	103,309
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	239,193
Available-for-sale Securities, Due after ten years, Amortized Cost	586,513
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	925,400
Available-for-sale Securities, Due after one year through five years, Fair Value	103,939
Available-for-sale Securities, Due after five years through ten years, Fair Value	239,790
Available-for-sale Securities, Due after ten years, Fair Value	582,770
Available-for-sale securities	$ 1,851,899	$ 1,938,499
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.31%
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	0.88%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	0.90%
Available-for-sale Securities, Total Weighted-Average Yield	0.62%