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, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 1,945	$ 1,134
Due after one through five years	4,800	5,031
Due after five through ten years	47,807	74,745
Due after ten years	857,464	817,161
Total debt securities with a single maturity date	912,016	898,071
Total Investment Securities	1,732,279	1,740,420
Fair Value
Due in one year or less	1,965	1,136
Due after one through five years	4,932	5,141
Due after five through ten years	49,833	76,920
Due after ten years	905,587	855,505
Total debt securities with a single maturity date	962,317	938,702
Total Investment Securities	1,815,775	1,790,025
Amortized Cost
Due in one year or less	8,740	9,920
Due after one through five years	33,393	45,197
Due after five through ten years	107,482	84,153
Due after ten years	245,414	231,964
Total debt securities with a single maturity date	395,029	371,234
Total Investment Securities	882,179	806,038
Fair Value
Due in one year or less	8,762	10,105
Due after one through five years	33,947	45,654
Due after five through ten years	111,986	88,844
Due after ten years	258,344	241,639
Total debt securities with a single maturity date	413,039	386,242
Investment securities held to maturity	920,178	827,566
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	820,263	842,349
Total Investment Securities	820,263	842,349
Fair Value
Without single maturity date	853,458	851,323
Total Investment Securities	853,458	851,323
Amortized Cost
Without single maturity date	487,150	434,804
Total Investment Securities	487,150	434,804
Fair Value
Without single maturity date	507,139	441,324
Investment securities held to maturity	$ 507,139	$ 441,324