Fair value measurements (Inflation and Discount Rate) (Details) - Significant Unobservable Inputs - Nonrecurring Fair Value Measurements	Jun. 30, 2020	Jun. 30, 2019
Inflation rate
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.0221	0.0225
Credit- adjusted risk-free discount rate | Minimum
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.2500	0.1235
Credit- adjusted risk-free discount rate | Maximum
Nonrecurring Fair Value Measurements
Asset retirement obligations, measurement input	0.2500	0.1460