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Related party transactions - Additional Information (Details) - Fit for Life, LLC - USD ($)
$ in Millions
3 Months Ended
Oct. 11, 2016
Mar. 31, 2018
Mar. 31, 2017
Related Party Transaction [Line Items]      
Percentage of outstanding common stock 10.00%    
Annual base rent for sublease   $ 0.3  
Sublease contractual term   39 months  
Recognized sublease income   $ 0.1 $ 0.1
Estimated future sublease income   $ 0.5