Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 12,601
Due after one year through five years	34,771
Due after five year through ten years	69
Total Available-for-sale debt securities and restricted investments, amortized cost	50,846
Fair Value
Due in one year or less	12,431
Due after one year through five years	33,866
Due after five year through ten years	49
Fair Value	46,346	$ 76,245
Total Available-for-sale debt securities and restricted investments, fair value	49,738
Debt Securities
Amortized Cost
Amortized Cost	47,441
Fair Value
Fair Value	46,346
Restricted investments
Amortized Cost
Amortized Cost	3,405
Fair Value
Fair Value	$ 3,392