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Critical accounting estimates and judgments - Summary of Key Assumptions (Details)
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Disclosure of information for cash-generating units [line items]    
Future pricing increases as a percentage 0.11% 1.13%
Future cost increases as a percentage 3.11% 0.09%
Discount rates 8.43% 7.60%
Perpetuity growth rate 2.00% 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 0.00% 0.00%
Top of range    
Disclosure of information for cash-generating units [line items]    
Yield average growth rates 1.00% 0.00%