Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Dec. 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	$ 879,669
Available-for-sale Securities, Due after one year through five years, Amortized Cost	370,435
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	424,882
Available-for-sale Securities, Due after ten years, Amortized Cost	843,980
Available-for-sale Securities, Amortized Cost Basis	2,518,966	$ 2,779,047
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	879,137
Available-for-sale Securities, Due after one year through five years, Fair Value	370,089
Available-for-sale Securities, Due after five years through ten years, Fair Value	426,270
Available-for-sale Securities, Due after ten years, Fair Value	840,355
Available-for-sale securities	$ 2,515,851	$ 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	0.76%
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	1.39%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.19%
Available-for-sale Securities, Total Weighted-Average Yield	1.06%