Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 13,974
Due after one year through five years	95,423
Due after five year through ten years	3,757
Due after ten years through fifteen years	92
Amortized Cost	113,246
Amortized Cost	151,625	$ 177,280
Fair Value
Due in one year or less	13,944
Due after one year through five years	93,476
Due after five year through ten years	4,025
Due after ten years through fifteen years	58
Fair Value	111,503
Fair Value	149,698	181,361
Preferred stock
Amortized Cost
Securities without maturity date, amortized cost	33,923
Amortized Cost	33,923	36,048
Fair Value
Securities without maturity date, fair value	33,733
Fair Value	33,733	36,593
Restricted investments
Amortized Cost
Securities without maturity date, amortized cost	4,456
Amortized Cost	4,456	5,642
Fair Value
Securities without maturity date, fair value	4,462
Fair Value	$ 4,462	$ 5,636