Derivative instruments (Summary of derivatives outstanding) (Details)	Feb. 28, 2019 MBbls BritishThermalUnits $ / bbl $ / MMBTU	Dec. 31, 2018 MBbls BritishThermalUnits $ / bbl $ / gal $ / MMBTU	Feb. 28, 2018 $ / bbl
Oil swaps | Derivative Maturing in 2019 | Subsequent Event
Derivative [Line Items]
Volume | MBbls	106
Weighted average fixed price (in dollars per bbl)	55.59
Oil swaps | Derivative Maturing in 2019 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		2,322
Weighted average fixed price (in dollars per bbl)		56.20
Oil swaps | Derivative Maturing in 2020
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl)			46.26
Oil swaps | Derivative Maturing in 2020 | Subsequent Event
Derivative [Line Items]
Volume | MBbls	240
Weighted average fixed price (in dollars per bbl)	52.43
Oil swaps | Derivative Maturing in 2020 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		1,767
Weighted average fixed price (in dollars per bbl)		50.30
Oil swaps | Derivative Maturing in 2021
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl)			44.34
Oil swaps | Derivative Maturing in 2021 | Subsequent Event
Derivative [Line Items]
Volume | MBbls	146
Weighted average fixed price (in dollars per bbl)	53.31
Oil swaps | Derivative Maturing in 2021 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		544
Weighted average fixed price (in dollars per bbl)		44.34
Oil roll swaps | Derivative Maturing in 2019 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		530
Weighted average fixed price (in dollars per bbl)		0.52
Oil roll swaps | Derivative Maturing in 2020 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		410
Weighted average fixed price (in dollars per bbl)		0.38
Oil roll swaps | Derivative Maturing in 2021 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		150
Weighted average fixed price (in dollars per bbl)		0.30
Natural gas swaps | Derivative Maturing in 2019 | Subsequent Event
Derivative [Line Items]
Volume | BritishThermalUnits	4,800
Weighted average fixed price (in dollars per bbl) | $ / MMBTU	2.86
Natural gas swaps | Derivative Maturing in 2019 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume | BritishThermalUnits		9,461
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		2.85
Weighted average fixed price per Bbl, purchased puts		0
Weighted average fixed price per Bbl, sold calls		0
Natural gas swaps | Derivative Maturing in 2020 | Subsequent Event
Derivative [Line Items]
Volume | BritishThermalUnits	2,400
Weighted average fixed price (in dollars per bbl) | $ / MMBTU	2.73
Natural gas swaps | Derivative Maturing in 2020 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume | BritishThermalUnits		3,600
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		2.77
Weighted average fixed price per Bbl, purchased puts		0
Weighted average fixed price per Bbl, sold calls		0
Natural gas basis swaps | Derivative Maturing in 2019 | Subsequent Event
Derivative [Line Items]
Volume | BritishThermalUnits	5,790
Weighted average fixed price (in dollars per bbl) | $ / MMBTU	(0.56)
Natural gas basis swaps | Derivative Maturing in 2019 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume | BritishThermalUnits		5,701
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		(0.67)
Weighted average fixed price per Bbl, purchased puts		0
Weighted average fixed price per Bbl, sold calls		0
Natural gas basis swaps | Derivative Maturing in 2020 | Subsequent Event
Derivative [Line Items]
Volume | BritishThermalUnits	3,600
Weighted average fixed price (in dollars per bbl) | $ / MMBTU	(0.46)
Natural gas collars | Derivative Maturing in 2019 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume | BritishThermalUnits		240
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		0.00
Weighted average fixed price per Bbl, purchased puts		4
Weighted average fixed price per Bbl, sold calls		5.07
Natural gasoline swaps | Derivative Maturing in 2019 | NGL derivative contracts
Derivative [Line Items]
Volume | MBbls		4,956
Weighted average fixed price (in dollars per bbl) | $ / gal		1.39
Natural gasoline swaps | Derivative Maturing in 2020 | NGL derivative contracts
Derivative [Line Items]
Volume | MBbls		1,890
Weighted average fixed price (in dollars per bbl) | $ / gal		1.39
Propane swaps | Derivative Maturing in 2019 | NGL derivative contracts
Derivative [Line Items]
Volume | MBbls		11,466
Weighted average fixed price (in dollars per bbl) | $ / gal		0.74
Propane swaps | Derivative Maturing in 2020 | NGL derivative contracts
Derivative [Line Items]
Volume | MBbls		4,284
Weighted average fixed price (in dollars per bbl) | $ / gal		0.74