Investment Securities - Debt Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 807,395
Available-for-sale Securities, Due after one year through five years, Amortized Cost	277,030
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	508,970
Available-for-sale Securities, Due after ten years, Amortized Cost	633,913
Available-for-sale Securities, Amortized Cost Basis	2,227,308	$ 2,216,953
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	806,214
Available-for-sale Securities, Due after one year through five years, Fair Value	277,382
Available-for-sale Securities, Due after five years through ten years, Fair Value	508,629
Available-for-sale Securities, Due after ten years, Fair Value	631,777
Available-for-sale securities	$ 2,224,002	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	1.24%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	2.52%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	2.46%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.65%
Available-for-sale Securities, Total Weighted-Average Yield	2.08%
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	400	2,589
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale securities	422	4,819
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-maturity Securities, Fair Value	$ 45,875	$ 45,564
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	3.30%	2.50%