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Summary of Significant Accounting Policies - Liquidity (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Accumulated deficit $ 151,864   $ 139,724  
Net cash used in operating activities 8,202 $ 13,494    
Cash and cash equivalents 14,004 $ 18,500 $ 22,408 $ 27,900
Debt covenant minimum cash balance $ 5,000      
Minimum        
Revenue Target [Line Items]        
Quarterly increase in revenue target (percent) 2.00%      
Maximum        
Revenue Target [Line Items]        
Quarterly increase in revenue target (percent) 4.00%