Revenue Recognition - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit	$ (328,026,000)		$ (328,026,000)		$ (315,065,000)
Revenues	13,538,000	$ 10,347,000	27,580,000	$ 18,317,000
Impairment charges related to contract assets			0
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit	55,000		55,000		$ (4,100,000)
Revenues	$ (1,602,000)		$ (5,440,000)