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, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 1,947	$ 1,134
Due after one through five years	4,589	5,031
Due after five through ten years	49,425	74,745
Due after ten years	976,970	817,161
Total debt securities with a single maturity date	1,032,931	898,071
Total Investment Securities	1,794,322	1,740,420
Fair Value
Due in one year or less	1,961	1,136
Due after one through five years	4,718	5,141
Due after five through ten years	52,485	76,920
Due after ten years	1,041,195	855,505
Total debt securities with a single maturity date	1,100,359	938,702
Total Investment Securities	1,890,593	1,790,025
Amortized Cost
Due in one year or less	12,191	9,920
Due after one through five years	23,639	45,197
Due after five through ten years	95,694	84,153
Due after ten years	254,290	231,964
Total debt securities with a single maturity date	385,814	371,234
Total Investment Securities	898,786	806,038
Fair Value
Due in one year or less	12,217	10,105
Due after one through five years	24,224	45,654
Due after five through ten years	101,354	88,844
Due after ten years	271,758	241,639
Total debt securities with a single maturity date	409,553	386,242
Investment securities held to maturity	941,426	827,566
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	761,391	842,349
Total Investment Securities	761,391	842,349
Fair Value
Without single maturity date	790,234	851,323
Total Investment Securities	790,234	851,323
Amortized Cost
Without single maturity date	512,972	434,804
Total Investment Securities	512,972	434,804
Fair Value
Without single maturity date	531,873	441,324
Investment securities held to maturity	$ 531,873	$ 441,324