Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for Sale, Amortized Cost
Due in one year or less	$ 425
Due after one through five years	5,040
Due after five through ten years	74,921
Due after ten years	430,497
Amortized Cost	510,883
Total Investment Securities	986,565
Available for Sale, Fair Value
Due in one year or less	425
Due after one through five years	5,204
Due after five through ten years	78,806
Due after ten years	442,289
Fair Value	526,724
Total Investment Securities	999,947
Held to Maturity, Amortized Cost
Due in one year or less	12,015
Due after one through five years	76,146
Due after five through ten years	54,441
Due after ten years	116,610
Amortized Cost	259,212
Amortized Cost	560,655	$ 607,643
Held to Maturity, Fair Value
Due in one year or less	12,158
Due after one through five years	76,334
Due after five through ten years	55,679
Due after ten years	120,657
Fair Value	264,828
Total Investment Securities	568,208	611,933
U.S. Government-sponsored mortgage-backed securities
Available for Sale, Amortized Cost
Without single maturity date	473,325
Available for Sale, Fair Value
Without single maturity date	470,866
Held to Maturity, Amortized Cost
Without single maturity date	301,443
Amortized Cost	301,443	360,213
Held to Maturity, Fair Value
Without single maturity date	303,380
Total Investment Securities	303,380	$ 363,642
Equity securities
Available for Sale, Amortized Cost
Without single maturity date	2,357
Available for Sale, Fair Value
Without single maturity date	$ 2,357