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Commitments and contingencies - Litigation (Details)
$ in Millions
Jun. 30, 2018
USD ($)
bbl
Jun. 15, 2018
USD ($)
Dec. 11, 2017
Claim
Commitments and Contingencies Disclosure [Abstract]      
Barrels of crude oil produced | bbl 19,000    
Number of causes of action | Claim     9
Gain Contingency, Unrecorded Amount   $ 150.0  
Estimated litigation liability $ 37.4