Acquisitions and divestitures Acquisitions and Divestitures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended
	Jun. 30, 2014 Divestiture	Jun. 30, 2014 Divestiture	Jun. 30, 2014 Andrews County, TX and Osage County, OK	Aug. 14, 2014 Subsequent Event Divestiture	Jul. 31, 2014 Subsequent Event Divestiture	May 31, 2014 Scout Energy Group I, LP Fort Worth Basin	May 31, 2014 RKI Exploration And Production, LLC Delaware Basin	Jul. 31, 2014 RAM Energy, LLC Subsequent Event Ark-La-Tex	Jul. 31, 2014 RAM Energy, LLC Subsequent Event Central Basin
Significant Acquisitions and Disposals [Line Items]
Number of divestitures closed	2			2	4
Number of planned divestitures		5
Proceeds from sale			$ 20,022			$ 25,150	$ 125,000	$ 48,495	$ 45,854