Investment Securities - Debt Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 799,132
Available-for-sale Securities, Due after one year through five years, Amortized Cost	339,562
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	319,231
Available-for-sale Securities, Due after ten years, Amortized Cost	759,028
Available-for-sale Securities, Amortized Cost Basis	2,216,953	$ 2,518,966
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	797,762
Available-for-sale Securities, Due after one year through five years, Fair Value	338,157
Available-for-sale Securities, Due after five years through ten years, Fair Value	320,961
Available-for-sale Securities, Due after ten years, Fair Value	758,525
Available-for-sale securities	$ 2,215,405	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	1.02%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.78%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.87%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.93%
Available-for-sale Securities, Total Weighted-Average Yield	1.57%
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 45,032
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	2,589	3,120
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale securities	4,819	$ 7,625
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-maturity Securities, Fair Value	$ 45,564
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	2.50%