Derivative instruments (Summary of Derivatives Outstanding) (Details)	Sep. 30, 2019 bbtu bbl gal $ / BTU $ / bbl $ / gal
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	688
Weighted average fixed price (in usd per unit)	55.90
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil swaps | Derivative Maturing In 2019 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbl	2,274
Weighted average fixed price (in usd per unit)	51.01
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in 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 (in usd per unit)	46.24
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in 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	120
Weighted average fixed price (in usd per unit)	0.46
Oil roll swaps | Derivative Maturing In 2019 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2019 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in 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 (in usd per unit)	0.38
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in 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 (in usd per unit)	0.30
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	0.00
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in 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 (in usd per unit)	0.00
Oil collars | Derivative Maturing In 2020 | Crude oil derivatives | Purchased Puts
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	55.00
Oil collars | Derivative Maturing In 2020 | Crude oil derivatives | Sold Calls
Derivative [Line Items]
Weighted average fixed price (in usd per unit)	66.42
Natural gas swaps | Derivative Maturing In 2019 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbtu	3,977
Weighted average fixed price (in 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) | bbtu	7,680
Weighted average fixed price (in usd per unit) | $ / BTU	2.70
Natural gas basis swaps | Derivative Maturing In 2019 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbtu	3,977
Weighted average fixed price (in usd per unit) | $ / BTU	0.51
Natural gas basis swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | bbtu	7,080
Weighted average fixed price (in 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	4,200
Weighted average fixed price (in 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	6,508
Weighted average fixed price (in usd per unit) | $ / gal	1.15
Propane swaps | Derivative Maturing In 2019 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	9,156
Weighted average fixed price (in 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	14,872
Weighted average fixed price (in usd per unit) | $ / gal	0.57
Butane swaps | Derivative Maturing In 2019 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	2,394
Weighted average fixed price (in usd per unit) | $ / gal	0.71
Butane swaps | Derivative Maturing In 2020 | Natural gas liquid derivatives
Derivative [Line Items]
Volume (in MBbl, BBtu, or Gallon) | gal	2,849
Weighted average fixed price (in usd per unit) | $ / gal	0.68