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Revenue recognition - Performance Obligations (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01    
Remaining Performance Obligation    
Deferred consideration payable   $ 11.7
Revenue, performance obligation, period   1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01    
Remaining Performance Obligation    
Deferred consideration payable $ 4.2  
Revenue, performance obligation, period 1 year