Derivative instruments (Summary of Derivatives Outstanding) (Details) MMBTU in Thousands		12 Months Ended
	Feb. 28, 2018 $ / bbl MBbls	Dec. 31, 2019 MMBTU $ / bbl $ / MMBTU MBbls gal
Oil swaps | Derivative Maturing in 2020
Derivative [Line Items]
Volume | MBbls	543
Weighted average fixed price (in dollars per bbl)	46.26
Oil swaps | Derivative Maturing in 2020 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		2,274
Weighted average fixed price (in dollars per bbl)		51.01
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 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		689
Weighted average fixed price (in dollars per bbl)		46.24
Oil collars | Derivative Maturing in 2020 | Crude Oil Derivative Contracts
Derivative [Line Items]
Volume | MBbls		195
Weighted average fixed price (in dollars per bbl)		0.00
Weighted average fixed price per Bbl, purchased puts		55.00
Weighted average fixed price per Bbl, sold calls		66.42
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 2020 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume BBtu | MMBTU		7,680
Natural gas basis swaps | Derivative Maturing in 2020 | Natural Gas Derivative Contracts
Derivative [Line Items]
Volume BBtu | MMBTU		7,080
Butane | Derivative Maturing in 2020 | NGL derivative contracts
Derivative [Line Items]
Volume | gal		2,849
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		0.68
Natural gasoline swaps | Derivative Maturing in 2020 | NGL derivative contracts
Derivative [Line Items]
Volume | gal		6,508
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		1.15
Propane swaps | Derivative Maturing in 2020 | NGL derivative contracts
Derivative [Line Items]
Volume | gal		14,872
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		0.57
Long | Natural gas swaps | Derivative Maturing in 2020 | Natural Gas Derivative Contracts
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		2.71
Short | Natural gas basis swaps | Derivative Maturing in 2020 | Natural Gas Derivative Contracts
Derivative [Line Items]
Weighted average fixed price (in dollars per bbl) | $ / MMBTU		0.46