Fair value measurements (Nonrecurring fair value measurements) (Details) - Level 3 - Nonrecurring Fair Value Measurements	Dec. 31, 2018	Dec. 31, 2017
Measurement Input, Inflation Rate
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.0226	0.0230
Measurement Input, Risk Free Interest Rate
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input		0.0513
Measurement Input, Risk Free Interest Rate | Minimum
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.0692
Measurement Input, Risk Free Interest Rate | Maximum
Nonrecurring Fair Value Measurements
Asset retirement obligation, measurement input	0.1194	0.0763