Reduction in Workforce (Details) - USD ($) $ in Thousands			12 Months Ended
	Sep. 20, 2016	Jun. 16, 2016	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring costs		$ 600
Restructuring charges	$ 1,300		$ 170
Expected cost			100
Restructuring Reserve [Roll Forward]
Beginning Balance			31
Charges	$ 1,300		170
Payments			(90)
Ending Balance			$ 111	$ 31
Research and development expense
Restructuring Cost and Reserve [Line Items]
Restructuring charges				1,100
Restructuring Reserve [Roll Forward]
Charges				1,100
General and administrative expense
Restructuring Cost and Reserve [Line Items]
Restructuring charges				800
Restructuring Reserve [Roll Forward]
Charges				$ 800