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Accounting policies - Performance obligations (Details)
Sep. 30, 2021
USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue, Remaining Performance Obligation, Amount $ 445,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period 3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue, Remaining Performance Obligation, Amount $ 6,400,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue, Remaining Performance Obligation, Amount $ 6,000,000.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period 2 years