Acquisitions and divestitures (2017 Acquisitions and Divestitures) (Details) a in Thousands	1 Months Ended			3 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Mar. 21, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) well	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Proceeds from asset dispositions							$ 50,523,000
Loss on sale of assets				$ (206,000)	$ 25,996,000	$ 6,000	2,582,000
Non-core Oil and Gas Properties
Long Lived Assets Held-for-sale [Line Items]
Proceeds from asset dispositions								$ 9,200,000
Loss on sale of assets							$ 0
Osage County Oklahoma
Long Lived Assets Held-for-sale [Line Items]
Proceeds from asset dispositions		$ 14,117,000
STACK
Long Lived Assets Held-for-sale [Line Items]
Payment to acquire property	$ 60,643,000
Area of land | a	7
Number of wells required to be completed in 2019 | well						10
Number of wells required to be completed in 2020 | well						10
Number of wells required to be completed in 2021 | well						10
Number of wells required to be completed in 2022 | well						15
Perdure Petroleum Limited Liability Company
Long Lived Assets Held-for-sale [Line Items]
Proceeds from asset dispositions			$ 163,630,000
Loss on sale of assets			$ 25,163,000