Oil and Natural Gas Property Transactions - VPP Transactions Table (Details) - VPP 9 Mid-Continent [Member] MBbls in Millions, $ in Millions, Mcfe in Billions, Mcf in Billions	May 31, 2011 USD ($) Mcfe MBbls Mcf
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $	$ 853
Proved Developed Reserves (Energy) | Mcfe	177
Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)	1.7
Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf	138
NGL [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)	4.8