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 ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Proceeds from asset dispositions						$ 50,523,000
Loss on sale of assets					$ 25,996,000	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	7,000
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	$ (25,163,000)