Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available for sale, Amortized Cost
Due in one year or less	$ 1,571
Due after one year through five years	67,794
Due after five years through ten years	127,156
Due after ten years	94,050
Agency mortgage-backed securities	911,770
Amortized Cost	1,202,341	$ 733,279
Available for sale, Estimated Fair Value
Due in one year or less	1,579
Due after one year through five years	68,369
Due after five years through ten years	131,019
Due after ten years	98,038
Agency mortgage-backed securities	924,047
Available for sale, fair value	1,223,052	721,369
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	3,883
Due after five years through ten years	8,591
Due after ten years	0
Agency mortgage-backed securities	50,251
Amortized Cost	62,725	65,679
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	3,931
Due after five years through ten years	8,702
Due after ten years	0
Agency mortgage-backed securities	50,428
Held to maturity, fair value	$ 63,061	$ 63,934