Derivative instruments (Summary of Derivatives Outstanding) (Details)	Jun. 30, 2019 BritishThermalUnits gal bbl $ / BTU $ / bbl $ / gal
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	1,311
Weighted average fixed price (usd per unit)	55.92
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	2,007
Weighted average fixed price (usd per unit)	50.56
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	689
Weighted average fixed price (usd per unit)	46.24
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2019 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	240
Weighted average fixed price (usd per unit)	0.46
Oil roll swaps | Derivative Maturing In 2019 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2019 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	410
Weighted average fixed price (usd per unit)	0.38
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	150
Weighted average fixed price (usd per unit)	0.30
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	0.00
Oil collars | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	195
Weighted average fixed price (usd per unit)	0.00
Oil collars | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (usd per unit)	55.00
Oil collars | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (usd per unit)	66.42
Natural gas swaps | Derivative Maturing In 2019 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | BritishThermalUnits	7,824
Weighted average fixed price (usd per unit) | $ / BTU	2.85
Natural gas swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | BritishThermalUnits	6,000
Weighted average fixed price (usd per unit) | $ / BTU	2.75
Natural gas basis swaps | Derivative Maturing In 2019 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | BritishThermalUnits	4,994
Weighted average fixed price (usd per unit) | $ / BTU	0.59
Natural gas basis swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | BritishThermalUnits	3,600
Weighted average fixed price (usd per unit) | $ / BTU	0.46
Natural gasoline swaps | Derivative Maturing In 2019 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	7,896
Weighted average fixed price (usd per unit) | $ / gal	1.13
Natural gasoline swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	4,788
Weighted average fixed price (usd per unit) | $ / gal	1.17
Propane swaps | Derivative Maturing In 2019 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	17,472
Weighted average fixed price (usd per unit) | $ / gal	0.61
Propane swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	10,668
Weighted average fixed price (usd per unit) | $ / gal	0.63
Butane | Derivative Maturing In 2019 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	4,536
Weighted average fixed price (usd per unit) | $ / gal	0.71
Butane | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	2,352
Weighted average fixed price (usd per unit) | $ / gal	0.71