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Revenue recognition - Performance Obligations (Details) - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Remaining Performance Obligation    
Deferred consideration payable $ 0  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01    
Remaining Performance Obligation    
Deferred consideration payable   $ 4,200,000
Revenue, performance obligation, period   9 months