Commitments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Rent expense	$ 1,300	$ 1,100	$ 685
Clinical trial master agreement expense	$ 27	$ 2,100	$ 1,600