Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 1,134	$ 13,092
Due after one through five years	4,527	5,311
Due after five through ten years	83,254	73,280
Due after ten years	714,515	527,835
Total debt securities with a single maturity date	803,430	619,518
Total Investment Securities	1,593,317	1,149,727
Fair Value
Due in one year or less	1,139	13,144
Due after one through five years	4,638	5,378
Due after five through ten years	85,475	74,411
Due after ten years	754,631	526,815
Total debt securities with a single maturity date	845,883	619,748
Total Investment Securities	1,646,399	1,142,195
Amortized Cost
Due in one year or less	13,833	4,732
Due after one through five years	55,424	49,054
Due after five through ten years	100,501	62,873
Due after ten years	229,439	104,868
Total debt securities with a single maturity date	399,197	221,527
Total Investment Securities	843,021	490,387
Fair Value
Due in one year or less	13,894	4,730
Due after one through five years	55,783	48,473
Due after five through ten years	104,733	64,163
Due after ten years	240,490	105,601
Total debt securities with a single maturity date	414,900	222,967
Investment securities held to maturity	865,286	489,217
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	789,887	530,209
Total Investment Securities	789,887	530,209
Fair Value
Without single maturity date	800,516	522,447
Total Investment Securities	800,516	522,447
Amortized Cost
Without single maturity date	443,824	268,860
Total Investment Securities	443,824	268,860
Fair Value
Without single maturity date	450,386	266,250
Investment securities held to maturity	$ 450,386	$ 266,250