Natural Gas and Oil Property Transactions - VPP Transactions Table (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended
	Dec. 31, 2014 Mcfe	Mar. 31, 2012 Mcfe	Jun. 30, 2011	Sep. 30, 2010 Mcfe	Dec. 31, 2008 Mcfe	Sep. 30, 2008	Jun. 30, 2008	Dec. 31, 2007 Mcfe	May 31, 2011 Mcfe	Aug. 31, 2008 Mcfe	May 31, 2008 Mcfe
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)	$ 5,481
Proved Developed Reserves (Energy)	1,220,000,000,000
Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)	5,200,000
Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)	1,105,000,000,000
NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)	14,000,000
VPP 10 Aradarko Basin Granite Wash [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)		744
Proved Developed Reserves (Energy)		160,000,000,000
VPP 10 Aradarko Basin Granite Wash [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)		3,000,000
VPP 10 Aradarko Basin Granite Wash [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)		87,000,000,000
VPP 10 Aradarko Basin Granite Wash [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)		9,200,000
VPP 9 Mid-Continent [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)			853
Proved Developed Reserves (Energy)									177,000,000,000
VPP 9 Mid-Continent [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)									1,700,000
VPP 9 Mid-Continent [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)									138,000,000,000
VPP 9 Mid-Continent [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)									4,800,000
VPP 8 Barnett Shale [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)				1,150
Proved Developed Reserves (Energy)				390,000,000,000
VPP 8 Barnett Shale [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)				0
VPP 8 Barnett Shale [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)				390,000,000,000
VPP 8 Barnett Shale [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)				0
VPP 4 Anadarko and Arkoma Basins [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)					412
Proved Developed Reserves (Energy)					98,000,000,000
VPP 4 Anadarko and Arkoma Basins [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)					500,000
VPP 4 Anadarko and Arkoma Basins [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)					95,000,000,000
VPP 4 Anadarko and Arkoma Basins [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)					0
VPP 3 Anadarko Basin [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)						600
Proved Developed Reserves (Energy)										93,000,000,000
VPP 3 Anadarko Basin [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)										0
VPP 3 Anadarko Basin [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)										93,000,000,000
VPP 3 Anadarko Basin [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)										0
VPP 2 Texas, Oklahoma and Kansas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)							622
Proved Developed Reserves (Energy)											94,000,000,000
VPP 2 Texas, Oklahoma and Kansas [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)											0
VPP 2 Texas, Oklahoma and Kansas [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)											94,000,000,000
VPP 2 Texas, Oklahoma and Kansas [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)											0
VPP 1 Kentucky and West Virginia [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Proceeds from Volumetric Production Payment (VPP)								$ 1,100
Proved Developed Reserves (Energy)								208,000,000,000
VPP 1 Kentucky and West Virginia [Member] | Oil [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)								0
VPP 1 Kentucky and West Virginia [Member] | Natural Gas [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)								208,000,000,000
VPP 1 Kentucky and West Virginia [Member] | NGL [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Proved Developed Reserves (Volume)								0