Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments:
Money market funds	$ 20,046	$ 68,234
Restricted cash	253	390
Total Financial Instruments	20,299	68,624
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Instruments:
Money market funds	20,046	68,234
Restricted cash	253	390
Total Financial Instruments	20,299	68,624
Significant Other Observable Inputs (Level 2)
Financial Instruments:
Money market funds	0	0
Restricted cash	0	0
Total Financial Instruments	0	0
Significant Unobservable Inputs (Level 3)
Financial Instruments:
Money market funds	0	0
Restricted cash	0	0
Total Financial Instruments	$ 0	$ 0