Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available for sale, Amortized Cost
Due in one year or less	$ 2,018
Due after one year through five years	26,301
Due after five years through ten years	10,205
Due after ten years	225,272
Agency mortgage-backed securities	863,698
Amortized Cost	1,127,494	$ 1,115,323
Available for sale, Estimated Fair Value
Due in one year or less	2,054
Due after one year through five years	27,369
Due after five years through ten years	10,576
Due after ten years	234,848
Agency mortgage-backed securities	886,667
Available for sale, fair value	1,161,514	1,135,317
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	4,444
Due after five years through ten years	130,071
Due after ten years	0
Agency mortgage-backed securities	44,720
Amortized Cost	179,235	181,166
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	4,503
Due after five years through ten years	126,934
Due after ten years	0
Agency mortgage-backed securities	46,644
Held to maturity, fair value	$ 178,081	$ 181,939