Investment Securities - Debt Maturities (Details) - USD ($) $ in Thousands	Jun. 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	$ 890,555
Available-for-sale Securities, Due after one year through five years, Amortized Cost	234,693
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	520,939
Available-for-sale Securities, Due after ten years, Amortized Cost	681,422
Available-for-sale Securities, Amortized Cost Basis	2,327,609	$ 2,216,953
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	889,077
Available-for-sale Securities, Due after one year through five years, Fair Value	234,979
Available-for-sale Securities, Due after five years through ten years, Fair Value	521,199
Available-for-sale Securities, Due after ten years, Fair Value	679,343
Available-for-sale securities	$ 2,324,598	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	1.18%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	2.40%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	2.35%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.41%
Available-for-sale Securities, Total Weighted-Average Yield	1.92%
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	416	2,589
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale securities	439	4,819
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-maturity Securities, Fair Value	$ 45,863	$ 45,564
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	3.00%	2.50%