Derivative instruments (Summary of Derivatives Outstanding) (Details)	Mar. 31, 2020 gal MMBTU bbl $ / bbl $ / gal $ / BTU
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	1,770
Weighted average fixed price (in usd per unit) | $ / bbl	51.16
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	689
Weighted average fixed price (in usd per unit) | $ / bbl	46.24
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	290
Weighted average fixed price (in usd per unit) | $ / bbl	0.34
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	150
Weighted average fixed price (in usd per unit) | $ / bbl	0.30
Natural gas swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | MMBTU	5,340
Weighted average fixed price (in usd per unit) | $ / BTU	2.72
Natural gas basis swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | MMBTU	5,040
Weighted average fixed price (in usd per unit) | $ / BTU	0.46
Natural gasoline swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | gal	2,476
Weighted average fixed price (in usd per unit) | $ / gal	1.17
Propane swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | gal	5,884
Weighted average fixed price (in usd per unit) | $ / gal	0.51
Butane swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | gal	497
Weighted average fixed price (in usd per unit) | $ / gal	0.53