Oil and Natural Gas Property Transactions - VPP Transactions Table (Details) Mcfe in Millions, Mcf in Millions, MBbls in Millions, $ in Millions	3 Months Ended							6 Months Ended
	Mar. 31, 2012 USD ($) Mcfe MBbls Mcf	Jun. 30, 2011 USD ($)	Sep. 30, 2010 USD ($) Mcfe MBbls Mcf	Dec. 31, 2008 USD ($) Mcfe MBbls Mcf	Sep. 30, 2008 USD ($)	Jun. 30, 2008 USD ($)	Dec. 31, 2007 USD ($) Mcfe MBbls Mcf	Jun. 30, 2015 USD ($) Mcfe MBbls Mcf	May. 31, 2011 Mcfe MBbls Mcf	Aug. 31, 2008 Mcfe MBbls Mcf	May. 31, 2008 Mcfe MBbls Mcf
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $								$ 5,481
Proved Developed Reserves (Energy) | Mcfe								1,220
Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)								5.2
Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf								1,105
Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)								14.0
VPP 10 Aradarko Basin Granite Wash [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $	$ 744
Proved Developed Reserves (Energy) | Mcfe	160
VPP 10 Aradarko Basin Granite Wash [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)	3.0
VPP 10 Aradarko Basin Granite Wash [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf	87
VPP 10 Aradarko Basin Granite Wash [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)	9.2
VPP 9 Mid-Continent [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $		$ 853
Proved Developed Reserves (Energy) | Mcfe									177
VPP 9 Mid-Continent [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)									1.7
VPP 9 Mid-Continent [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf									138
VPP 9 Mid-Continent [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)									4.8
VPP 8 Barnett Shale [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $			$ 1,150
Proved Developed Reserves (Energy) | Mcfe			390
VPP 8 Barnett Shale [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)			0.0
VPP 8 Barnett Shale [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf			390
VPP 8 Barnett Shale [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)			0.0
VPP 4 Anadarko and Arkoma Basins [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $				$ 412
Proved Developed Reserves (Energy) | Mcfe				98
VPP 4 Anadarko and Arkoma Basins [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)				0.5
VPP 4 Anadarko and Arkoma Basins [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf				95
VPP 4 Anadarko and Arkoma Basins [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)				0.0
VPP 3 Anadarko Basin [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $					$ 600
Proved Developed Reserves (Energy) | Mcfe										93
VPP 3 Anadarko Basin [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)										0.0
VPP 3 Anadarko Basin [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf										93
VPP 3 Anadarko Basin [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)										0.0
VPP 2 Texas, Oklahoma and Kansas [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $						$ 622
Proved Developed Reserves (Energy) | Mcfe											94
VPP 2 Texas, Oklahoma and Kansas [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)											0.0
VPP 2 Texas, Oklahoma and Kansas [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf											94
VPP 2 Texas, Oklahoma and Kansas [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)											0.0
VPP 1 Kentucky and West Virginia [Member]
VPP Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP) | $							$ 1,100
Proved Developed Reserves (Energy) | Mcfe							208
VPP 1 Kentucky and West Virginia [Member] | Oil [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)							0.0
VPP 1 Kentucky and West Virginia [Member] | Natural Gas [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume) | Mcf							208
VPP 1 Kentucky and West Virginia [Member] | Natural Gas Liquids [Member]
VPP Transactions [Line Items]
Proved Developed Reserves (Volume)							0.0