Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for Sale, Amortized Cost
Due in one year or less	$ 1,349	$ 1,134
Due after one through five years	5,545	5,031
Due after five through ten years	70,777	74,745
Due after ten years	1,097,451	817,161
Amortized Cost	1,175,122	898,071
Amortized Cost	1,807,389	1,740,420
Available for Sale, Fair Value
Due in one year or less	1,353	1,136
Due after one through five years	5,764	5,141
Due after five through ten years	75,223	76,920
Due after ten years	1,182,110	855,505
Fair Value	1,264,450	938,702
Total Investment Securities	1,919,119	1,790,025
Held to Maturity, Amortized Cost
Due in one year or less	9,712	9,920
Due after one through five years	22,241	45,197
Due after five through ten years	115,408	84,153
Due after ten years	505,153	231,964
Amortized Cost	652,514	371,234
Amortized Cost	1,227,668	806,038
Held to Maturity, Fair Value
Due in one year or less	9,755	10,105
Due after one through five years	23,190	45,654
Due after five through ten years	121,333	88,844
Due after ten years	533,079	241,639
Fair Value	687,357	386,242
Total Investment Securities	1,280,293	827,566
U.S. Government-sponsored mortgage-backed securities
Available for Sale, Amortized Cost
U.S. Government-sponsored mortgage-backed securities	632,267	842,349
Amortized Cost	632,267	842,349
Available for Sale, Fair Value
Without single maturity date	654,669	851,323
Total Investment Securities	654,669	851,323
Held to Maturity, Amortized Cost
Without single maturity date	575,154	434,804
Amortized Cost	575,154	434,804
Held to Maturity, Fair Value
Without single maturity date	592,936	441,324
Total Investment Securities	$ 592,936	$ 441,324