Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Jun. 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	$ 916,794
Available-for-sale Securities, Due after one year through five years, Amortized Cost	220,592
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	406,525
Available-for-sale Securities, Due after ten years, Amortized Cost	818,642
Available-for-sale Securities, Amortized Cost Basis	2,362,553	$ 2,518,966
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	915,808
Available-for-sale Securities, Due after one year through five years, Fair Value	220,936
Available-for-sale Securities, Due after five years through ten years, Fair Value	409,570
Available-for-sale Securities, Due after ten years, Fair Value	817,491
Available-for-sale securities	$ 2,363,805	$ 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	0.82%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.65%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.72%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.64%
Available-for-sale Securities, Total Weighted-Average Yield	1.34%