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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 26, 2017
Feb. 17, 2016
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Impact of 10% change in expected volatility rate     $ 18 $ 12  
Impact of 10% change in the estimate of fair value of the common stock     34 26  
Warrant Liability          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Liability fair value     $ 211 201 $ 0
Warrant Liability | Series C          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Expected term (in years)   5 years      
Risk-free interest rate (as a percent)   1.26%      
Expected volatility (as a percent)   62.99%      
Expected dividend yield (as a percent)   0.00%      
Warrant Liability | Common Stock          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Expected term (in years) 2 years   2 years    
Risk-free interest rate (as a percent) 1.44%   1.46%    
Expected volatility (as a percent) 78.71%   78.78%    
Expected dividend yield (as a percent) 0.00%   0.00%    
Liability fair value     $ 211 $ 0  
Subsequent Event | Warrant Liability          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Liability fair value $ 0