Investments - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available for sale, Amortized Cost
Due in one year or less	$ 2,675
Due after one year through five years	121,187
Due after five years through ten years	12,703
Due after ten years	5,044
Agency mortgage-backed securities	525,810
Amortized Cost	667,419	$ 647,141
Available for sale, Estimated Fair Value
Due in one year or less	2,690
Due after one year through five years	119,369
Due after five years through ten years	12,804
Due after ten years	4,988
Agency mortgage-backed securities	511,289
Available for sale, fair value	651,140	641,382
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	867
Due after five years through ten years	10,804
Due after ten years	866
Agency mortgage-backed securities	56,394
Amortized Cost	68,931	73,749
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	866
Due after five years through ten years	10,603
Due after ten years	835
Agency mortgage-backed securities	54,605
Held to maturity, fair value	$ 66,909	$ 73,458