Fair Value of Financial Instruments - Measurement of the Company's Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Money market funds	$ 15,050		$ 20,046
Liabilities:
Embedded derivative liability	902	$ 900
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Money market funds	15,050		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	$ 902