Fair value measurements (Nonrecurring Fair Value Measurements) (Details) - Level 3 - Nonrecurring Fair Value Measurements	Dec. 31, 2019	Dec. 31, 2018
Inflation rate | Low
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.0225	0.0226
Inflation rate | High
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.0225	0.0226
Credit adjusted risk-free discount rate | Low
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.1235	0.0692
Credit adjusted risk-free discount rate | High
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.2179	0.1194