Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for sale, Amortized Cost
Due in one year or less	$ 1,079
Due after one year through five years	27,564
Due after five years through ten years	127,062
Due after ten years	144,831
Agency mortgage-backed securities	917,102
Amortized Cost	1,217,638	$ 733,279
Available for sale, Estimated Fair Value
Due in one year or less	1,086
Due after one year through five years	28,081
Due after five years through ten years	133,705
Due after ten years	150,350
Agency mortgage-backed securities	934,111
Available for sale, fair value	1,247,333	721,369
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	3,879
Due after five years through ten years	8,589
Due after ten years	0
Agency mortgage-backed securities	48,318
Amortized Cost	60,786	65,679
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	3,929
Due after five years through ten years	8,706
Due after ten years	0
Agency mortgage-backed securities	48,988
Held to maturity, fair value	$ 61,623	$ 63,934