Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 28,886
Due after one year through five years	92,165
Due after five year through ten years	1,351
Due after ten years through fifteen years	91
Amortized Cost	122,493
Amortized Cost	173,972	$ 177,280
Fair Value
Due in one year or less	28,584
Due after one year through five years	90,963
Due after five year through ten years	1,339
Due after ten years through fifteen years	56
Fair Value	120,942
Fair Value	172,211	181,361
Preferred stock
Amortized Cost
Securities without maturity date, amortized cost	40,525
Amortized Cost	40,525	36,048
Fair Value
Securities without maturity date, fair value	40,277
Fair Value	40,277	36,593
Common stock
Amortized Cost
Securities without maturity date, amortized cost	6,481
Amortized Cost	6,481
Fair Value
Securities without maturity date, fair value	6,523
Fair Value	6,523
Restricted investments
Amortized Cost
Securities without maturity date, amortized cost	4,473
Amortized Cost	4,473	5,642
Fair Value
Securities without maturity date, fair value	4,469
Fair Value	$ 4,469	$ 5,636