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Summary of significant accounting policies - Net loss per share - (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Earnings Per Share, Pro Forma [Abstract]                      
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)                 5,394,249 4,723,719 8,461,169
Add: Changes in fair value of preferred stock tranche liability                   $ 9,750 $ 1,949
Net loss attributable to common stockholders $ (14,674) $ (8,996) $ (9,335) $ (7,188) $ (8,817) $ (6,914) $ (6,746) $ (15,813) $ (40,193) (38,290) (17,683)
Accretion of redeemable convertible preferred stock to redemption value                   (7,373) $ (1,366)
Add: Accrued dividends on temporary equity                   $ 1,245  
Weighted average number of common shares outstanding, basic and diluted (in shares)                 25,302,414 4,191,210 635,448
Redeemable convertible preferred stock                      
Earnings Per Share, Pro Forma [Abstract]                      
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)                     5,882,352
Unvested restricted stock awards                      
Earnings Per Share, Pro Forma [Abstract]                      
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)                 1,167,984 1,818,261 2,578,817
Outstanding stock options                      
Earnings Per Share, Pro Forma [Abstract]                      
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)                 4,226,265 2,905,458 0