Derivative instruments (Additional Information) (Details) - Oil swaps	Feb. 28, 2018 $ / bbl MBbls
Derivative Maturing in 2018
Derivative [Line Items]
Volume (in MBbls) | MBbls	1,086
Weighted average fixed price (in dollars per bbl)	54.80
Renegotiated Derivative Maturing in 2018
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl)	60.00
Derivative Maturing in 2020
Derivative [Line Items]
Volume (in MBbls) | MBbls	543
Weighted average fixed price (in dollars per bbl)	46.26
Derivative Maturing in 2021
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl)	44.34