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Summary of significant accounting policies - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]      
Deferred revenue $ 15 $ 15 $ 33
Deferred revenue realization period   12 months  
Revenue recognized, previously reported as deferred $ 6 $ 33