Investment Securities - Debt Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
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	$ 716,640
Available-for-sale Securities, Due after one year through five years, Amortized Cost	358,743
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	402,603
Available-for-sale Securities, Due after ten years, Amortized Cost	716,435
Available-for-sale Securities, Amortized Cost Basis	2,194,421	$ 2,216,953
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	715,000
Available-for-sale Securities, Due after one year through five years, Fair Value	358,688
Available-for-sale Securities, Due after five years through ten years, Fair Value	405,264
Available-for-sale Securities, Due after ten years, Fair Value	714,400
Available-for-sale securities	$ 2,193,352	2,215,405
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	1.09%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.83%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	2.06%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.19%
Available-for-sale Securities, Total Weighted-Average Yield	1.75%
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	45,032
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	2,464	2,589
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale securities	4,585	4,819
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-maturity Securities, Fair Value	$ 46,363	$ 45,564
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	2.80%	2.50%