Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 1,131	$ 13,092
Due after one through five years	3,752	5,311
Due after five through ten years	74,311	73,280
Due after ten years	639,454	527,835
Total debt securities with a single maturity date	718,648	619,518
Total Investment Securities	1,360,946	1,149,727
Fair Value
Due in one year or less	1,138	13,144
Due after one through five years	3,851	5,378
Due after five through ten years	76,604	74,411
Due after ten years	668,143	526,815
Total debt securities with a single maturity date	749,736	619,748
Total Investment Securities	1,399,959	1,142,195
Amortized Cost
Due in one year or less	3,784	4,732
Due after one through five years	46,941	49,054
Due after five through ten years	78,716	62,873
Due after ten years	216,000	104,868
Total debt securities with a single maturity date	345,441	221,527
Total Investment Securities	692,965	490,387
Fair Value
Due in one year or less	3,782	4,730
Due after one through five years	46,914	48,473
Due after five through ten years	81,769	64,163
Due after ten years	223,895	105,601
Total debt securities with a single maturity date	356,360	222,967
Investment securities held to maturity	708,915	489,217
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	642,298	530,209
Fair Value
Without single maturity date	650,223	522,447
Amortized Cost
Without single maturity date	347,524	268,860
Fair Value
Without single maturity date	$ 352,555	$ 266,250