Acquisitions and divestitures (2019 and 2018 Acquisitions and Divestitures) (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 21, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) a provider	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of leasehold acreage			$ 23,107,000
Proceeds from asset dispositions				$ 50,523,000
Gain (loss) on sale of assets	$ 206,000	$ (25,996,000)	(6,000)	(2,582,000)
Change in accrued oil and gas capital expenditures	$ 5,387,000	$ 9,534,000	$ 3,630,000	6,559,000
Other Non-core Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from asset dispositions				22,637,000
Non-core Oil and Gas Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from asset dispositions					$ 9,200,000
Gain (loss) on sale of assets				0
Oklahoma/Texas Panhandle
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from asset dispositions				17,000,000
Conveyance in liabilities to buyer				$ 629,000
Kingfisher and Garfield Counties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service contracts term				15 years
Kingfisher and Garfield Counties | Saltwater Disposal Infrastructure
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from asset dispositions				$ 11,841,000
Kingfisher, Garfield and Canadian Counties, Oklahoma
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of service providers | provider				2
Canadian County, Oklahoma
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service contracts term				5 years
STACK
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acquisitions of leasehold acreage				$ 122,309,000
Area of real estate property (in acres) | a				24,600
Capitalized interest				$ 10,925,000
Change in accrued oil and gas capital expenditures				$ 10,913,000