Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 798,153
Available-for-sale Securities, Due after one year through five years, Amortized Cost	363,861
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	279,192
Available-for-sale Securities, Due after ten years, Amortized Cost	791,792
Available-for-sale Securities, Amortized Cost Basis	2,232,998	$ 2,518,966
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	797,630
Available-for-sale Securities, Due after one year through five years, Fair Value	364,132
Available-for-sale Securities, Due after five years through ten years, Fair Value	281,792
Available-for-sale Securities, Due after ten years, Fair Value	791,412
Available-for-sale securities	$ 2,234,966	$ 2,515,851
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.92%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.62%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.75%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.86%
Available-for-sale Securities, Total Weighted-Average Yield	1.47%