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Critical accounting estimates and judgments - Narrative (Details) - cash_generating_unit
Sep. 30, 2017
Sep. 30, 2016
Disclosure of information for cash-generating units [line items]    
Number of cash-generating units 39 39
Argentina and Uruguay    
Disclosure of information for cash-generating units [line items]    
Number of cash-generating units 11 11
Brazil    
Disclosure of information for cash-generating units [line items]    
Number of cash-generating units 3 3