Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cost
Due in one year or less	$ 11,892
Due after one year through five years	65,713
Due after five year through ten years	68
Amortized Cost	77,673
Preferred stock	35,955
Restricted investments	4,482
Amortized Cost	118,110	$ 177,280
Fair Value
Due in one year or less	11,852
Due after one year through five years	64,332
Due after five year through ten years	61
Fair Value	76,245
Preferred stock	35,023
Restricted investments	4,469
Available-for-sale Securities	$ 115,737	$ 181,361