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Related party transactions - Additional Information (Details) - Fit for Life, LLC - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Related Party Transaction [Line Items]        
Percentage of outstanding common stock     8.00%  
Annual base rent for sublease     $ 0.3  
Sublease contractual term     39 months  
Recognized sublease income $ 0.1 $ 0.1 $ 0.2 $ 0.1
Estimated future sublease income $ 0.5   $ 0.5