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, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 4,746	$ 13,092
Due after one through five years	3,750	5,311
Due after five through ten years	79,533	73,280
Due after ten years	579,113	527,835
Total debt securities with a single maturity date	667,142	619,518
Total Investment Securities	1,229,729	1,149,727
Fair Value
Due in one year or less	4,764	13,144
Due after one through five years	3,835	5,378
Due after five through ten years	81,458	74,411
Due after ten years	595,535	526,815
Total debt securities with a single maturity date	685,592	619,748
Total Investment Securities	1,247,632	1,142,195
Amortized Cost
Due in one year or less	3,502	4,732
Due after one through five years	49,060	49,054
Due after five through ten years	65,990	62,873
Due after ten years	159,587	104,868
Total debt securities with a single maturity date	278,139	221,527
Total Investment Securities	615,027	490,387
Fair Value
Due in one year or less	3,501	4,730
Due after one through five years	48,717	48,473
Due after five through ten years	68,332	64,163
Due after ten years	163,804	105,601
Total debt securities with a single maturity date	284,354	222,967
Investment securities held to maturity	622,635	489,217
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	562,587	530,209
Fair Value
Without single maturity date	562,040	522,447
Amortized Cost
Without single maturity date	336,888	268,860
Fair Value
Without single maturity date	$ 338,281	$ 266,250