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, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 1,649	$ 1,134
Due after one through five years	4,571	5,031
Due after five through ten years	58,644	74,745
Due after ten years	1,016,232	817,161
Total debt securities with a single maturity date	1,081,096	898,071
Total Investment Securities	1,726,185	1,740,420
Fair Value
Due in one year or less	1,659	1,136
Due after one through five years	4,728	5,141
Due after five through ten years	61,968	76,920
Due after ten years	1,085,410	855,505
Total debt securities with a single maturity date	1,153,765	938,702
Total Investment Securities	1,824,160	1,790,025
Amortized Cost
Due in one year or less	9,677	9,920
Due after one through five years	25,872	45,197
Due after five through ten years	106,342	84,153
Due after ten years	454,241	231,964
Total debt securities with a single maturity date	596,132	371,234
Total Investment Securities	1,109,126	806,038
Fair Value
Due in one year or less	9,758	10,105
Due after one through five years	26,925	45,654
Due after five through ten years	112,055	88,844
Due after ten years	472,001	241,639
Total debt securities with a single maturity date	620,739	386,242
Investment securities held to maturity	1,152,518	827,566
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	645,089	842,349
Total Investment Securities	645,089	842,349
Fair Value
Without single maturity date	670,395	851,323
Total Investment Securities	670,395	851,323
Amortized Cost
Without single maturity date	512,994	434,804
Total Investment Securities	512,994	434,804
Fair Value
Without single maturity date	531,779	441,324
Investment securities held to maturity	$ 531,779	$ 441,324