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Critical accounting estimates and judgments - Key Assumptions (Details)
 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Disclosure of information for cash-generating units [line items]    
Future pricing increases (as a percent) 1.76% 1.76%
Future cost increases (as a percent) 0.33% 0.33%
Discount rates (as a percent) 4.00% 7.00%
Perpetuity growth rate (as a percent) 1.00% 1.00%
UMA    
Disclosure of information for cash-generating units [line items]    
Financial projections (in years) 5 years 4 years
AVI    
Disclosure of information for cash-generating units [line items]    
Financial projections (in years) 5 years 7 years
Bottom of range    
Disclosure of information for cash-generating units [line items]    
Yield average growth rates (as a percent) 0.00% 0.00%
Top of range    
Disclosure of information for cash-generating units [line items]    
Yield average growth rates (as a percent) 1.00% 1.00%