Fair Value of Financial Instruments - Measurement of the Company's Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Money market funds	$ 15,042	$ 20,046
Liabilities:
Embedded derivative liability	881
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Money market funds	15,042	20,046
Liabilities:
Embedded derivative liability	0
Significant Other Observable Inputs (Level 2)
Assets:
Money market funds	0	0
Liabilities:
Embedded derivative liability	0
Significant Unobservable Inputs (Level 3)
Assets:
Money market funds	0	$ 0
Liabilities:
Embedded derivative liability	$ 881