Fair value measurements (Inflation and Discount Rate) (Details) - Significant Unobservable Inputs - Nonrecurring Fair Value Measurements	Jun. 30, 2019	Jun. 30, 2018
Low | Inflation rate
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.0225	0.0226
Low | Credit-adjusted risk-free discount rate
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.1235	0.0692
High | Inflation rate
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.0225	0.0226
High | Credit-adjusted risk-free discount rate
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.1460	0.0868