Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available for sale, Amortized Cost
Due in one year or less	$ 11,345
Due after one year through five years	70,321
Due after five years through ten years	65,658
Due after ten years	453,611
Agency mortgage-backed securities	464,288
Amortized Cost	1,065,223	$ 882,682
Available for sale, Estimated Fair Value
Due in one year or less	11,435
Due after one year through five years	70,640
Due after five years through ten years	66,316
Due after ten years	458,453
Agency mortgage-backed securities	477,379
Available for sale, fair value	1,084,223	912,429
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	12,469
Due after five years through ten years	137,489
Due after ten years	216,612
Agency mortgage-backed securities	84,544
Amortized Cost	451,114	488,059
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	12,839
Due after five years through ten years	143,056
Due after ten years	217,451
Agency mortgage-backed securities	85,679
Held to maturity, fair value	$ 459,025	$ 501,523