Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Mar. 31, 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	$ 988,363
Available-for-sale Securities, Due after one year through five years, Amortized Cost	227,113
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	437,367
Available-for-sale Securities, Due after ten years, Amortized Cost	829,565
Available-for-sale Securities, Amortized Cost Basis	2,482,408	$ 2,518,966
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	987,343
Available-for-sale Securities, Due after one year through five years, Fair Value	227,471
Available-for-sale Securities, Due after five years through ten years, Fair Value	438,280
Available-for-sale Securities, Due after ten years, Fair Value	826,150
Available-for-sale securities	$ 2,479,244	$ 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	0.77%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.42%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.54%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.42%
Available-for-sale Securities, Total Weighted-Average Yield	1.18%