Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 544	$ 425
Due after one through five years	4,982	5,040
Due after five through ten years	81,890	74,921
Due after ten years	429,990	430,497
Total debt securities with a single maturity date	517,406	510,883
Total Investment Securities	1,014,230	986,565
Fair Value
Due in one year or less	549	425
Due after one through five years	5,064	5,204
Due after five through ten years	83,630	78,806
Due after ten years	427,520	442,289
Total debt securities with a single maturity date	516,763	526,724
Total Investment Securities	1,003,799	999,947
Amortized Cost
Due in one year or less	18,366	12,015
Due after one through five years	68,198	76,146
Due after five through ten years	52,976	54,441
Due after ten years	112,699	116,610
Total debt securities with a single maturity date	252,239	259,212
Total Investment Securities	540,021	560,655
Fair Value
Due in one year or less	18,675	12,158
Due after one through five years	67,756	76,334
Due after five through ten years	53,437	55,679
Due after ten years	113,523	120,657
Total debt securities with a single maturity date	253,391	264,828
Investment securities held to maturity	538,399	568,208
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	496,824	473,325
Fair Value
Without single maturity date	487,036	470,866
Amortized Cost
Without single maturity date	287,782	301,443
Fair Value
Without single maturity date	$ 285,008	303,380
Equity securities
Amortized Cost
Without single maturity date		2,357
Fair Value
Without single maturity date		2,357
Amortized Cost
Without single maturity date
Fair Value
Without single maturity date