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Summary of significant accounting policies - Additional Information 1 (Details)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-10-01  
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]  
Estimated revenue expected to be recognized in the future $ 154
Revenue, remaining performance obligation, expected timing of satisfaction, period 3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01  
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]  
Estimated revenue expected to be recognized in the future $ 567
Revenue, remaining performance obligation, expected timing of satisfaction, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2020-01-01  
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]  
Estimated revenue expected to be recognized in the future
Revenue, remaining performance obligation, expected timing of satisfaction, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: (nil)  
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]  
Estimated revenue expected to be recognized in the future $ 892
Period over which subscription contract terms exceed 12 months