Acquisitions and divestitures Acquisitions and Divestitures (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 31, 2014 Divestiture	May 31, 2014 Divestiture	Sep. 30, 2014 Divestiture	Sep. 30, 2014 Divestiture
Significant Acquisitions and Disposals [Line Items]
Number of planned divestitures				5
Number of divestitures closed	2	2	2
Andrews County, TX and Osage County, OK
Significant Acquisitions and Disposals [Line Items]
Proceeds from sale				$ 22,215
Scout Energy Group I, LP | Fort Worth Basin
Significant Acquisitions and Disposals [Line Items]
Price Of Divested Properties		23,702
RKI Exploration And Production, LLC | Delaware Basin
Significant Acquisitions and Disposals [Line Items]
Price Of Divested Properties		124,717
RAM Energy, LLC | Ark-La-Tex
Significant Acquisitions and Disposals [Line Items]
Price Of Divested Properties	48,495
RAM Energy, LLC | Central Basin
Significant Acquisitions and Disposals [Line Items]
Price Of Divested Properties	$ 45,854