Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for sale, Amortized Cost
Due in one year or less	$ 3,139
Due after one year through five years	32,281
Due after five years through ten years	16,828
Due after ten years	335,239
Agency mortgage-backed securities	581,111
Amortized Cost	968,598	$ 1,115,323
Available for sale, Estimated Fair Value
Due in one year or less	3,161
Due after one year through five years	33,232
Due after five years through ten years	17,443
Due after ten years	345,641
Agency mortgage-backed securities	605,949
Available for sale, fair value	1,005,426	1,135,317
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	10,187
Due after five years through ten years	125,476
Due after ten years	82,077
Agency mortgage-backed securities	109,954
Amortized Cost	327,694	181,166
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	10,587
Due after five years through ten years	133,063
Due after ten years	82,850
Agency mortgage-backed securities	112,274
Held to maturity, fair value	$ 338,774	$ 181,939