Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Mar. 31, 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	$ 940,691
Available-for-sale Securities, Due after one year through five years, Amortized Cost	188,476
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	545,148
Available-for-sale Securities, Due after ten years, Amortized Cost	783,559
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	941,643
Available-for-sale Securities, Due after one year through five years, Fair Value	188,801
Available-for-sale Securities, Due after five years through ten years, Fair Value	542,273
Available-for-sale Securities, Due after ten years, Fair Value	779,865
Available-for-sale securities	$ 2,452,582	$ 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	0.55%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.11%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.01%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.12%
Available-for-sale Securities, Total Weighted-Average Yield	0.88%