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Critical accounting estimates and judgments - Key Assumptions (Details)
 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Disclosure of information for cash-generating units [line items]    
Future pricing increases (as a percent) 0.11% 0.11%
Future cost increases (as a percent) 0.78% 3.11%
Discount rates (as a percent) 7.00% 8.00%
Perpetuity growth rate (as a percent) 1.00% 2.00%
UMA    
Disclosure of information for cash-generating units [line items]    
Financial projections (in years) 4 years 4 years
AVI    
Disclosure of information for cash-generating units [line items]    
Financial projections (in years) 7 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%