Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available for sale, Amortized Cost
Due in one year or less	$ 2,313
Due after one year through five years	25,981
Due after five years through ten years	9,455
Due after ten years	230,153
Agency mortgage-backed securities	693,082
Amortized Cost	960,984	$ 1,115,323
Available for sale, Estimated Fair Value
Due in one year or less	2,345
Due after one year through five years	27,058
Due after five years through ten years	9,915
Due after ten years	237,795
Agency mortgage-backed securities	720,991
Available for sale, fair value	998,104	1,135,317
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	10,207
Due after five years through ten years	126,285
Due after ten years	82,591
Agency mortgage-backed securities	127,353
Amortized Cost	346,436	181,166
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	10,597
Due after five years through ten years	133,206
Due after ten years	82,542
Agency mortgage-backed securities	129,794
Held to maturity, fair value	$ 356,139	$ 181,939