Acquisitions (Details) (USD $)	0 Months Ended		6 Months Ended
	Dec. 09, 2010 MW		Jun. 30, 2011 MW Tons Months	Sep. 30, 2010
CEG Acquisition [Abstract]
CEG Purchase Price			$ 7,900,000,000
CEG Plant Divestiture Wattage			2,648
CEG Transaction Costs			24,000,000
CEG Merger Integration Costs			500,000,000
Wolf Hollow Acquisiton [Abstract]
Wolf Hollow Acquisition Price			305,000,000
Wolf Hollow Acquisition Watts			720
GenCo PPA with Wolf Hollow			340,000,000
Acquisition of Exelon Wind [Abstract]
Acquisition of John Deere Renewables operating wind capacity	735
Contingent consideration potential cash payment	40,000,000
Acquisition of John Deere Renewables additional wind projects in advance stages of development capacity	230
Contingent consideration	(32,000,000)		32,000,000
Current portion of contingent consideration			24,000,000
Acquired customer receivables	18,000,000
Value of noncontrolling interest	3,000,000
Acquisition-related costs			24,000,000
Fair Value Of Consideration Transferred [Abstract]
Cash	893,000,000	[1]
Contingent consideration	32,000,000		(32,000,000)
Total fair value of consideration recorded	925,000,000
Recognized Amounts Of Identifiable Assets Acquired And Liabilities Assumed [Abstract]
Property, plant and equipment	700,000,000
Intangible assets	224,000,000
Working capital	18,000,000
Asset retirement obligation	(13,000,000)
Noncontrolling interest	(3,000,000)
Other	(1,000,000)
Total net identifiable assets	925,000,000
Footnotes To Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Net [Abstract]
Amount of long-term debt				900,000,000
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future Amortization Expense Year One			6,000,000
Future Amortization Expense Year Two			13,000,000
Future Amortization Expense Year Three			14,000,000
Future Amortization Expense Year Four			14,000,000
Future Amortization Expense Year Five			14,000,000
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross			224,000,000
Finite lived intangible assets accumulated amortization			(7,000,000)
Finite lived intangible assets net			217,000,000
Exelon Generation Co L L C [Member]
Acquisition of Exelon Wind [Abstract]
Acquisition of John Deere Renewables operating wind capacity	735
Contingent consideration potential cash payment	40,000,000
Acquisition of John Deere Renewables additional wind projects in advance stages of development capacity	230
Contingent consideration	(32,000,000)		32,000,000
Current portion of contingent consideration			24,000,000
Acquired customer receivables	18,000,000
Value of noncontrolling interest	3,000,000
Fair Value Of Consideration Transferred [Abstract]
Cash	893,000,000	[1]
Contingent consideration	32,000,000		(32,000,000)
Total fair value of consideration recorded	925,000,000
Recognized Amounts Of Identifiable Assets Acquired And Liabilities Assumed [Abstract]
Property, plant and equipment	700,000,000
Intangible assets	224,000,000
Working capital	18,000,000
Asset retirement obligation	(13,000,000)
Noncontrolling interest	(3,000,000)
Other	(1,000,000)
Total net identifiable assets	925,000,000
Footnotes To Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Net [Abstract]
Amount of long-term debt				900,000,000
Exelon Generation Co L L C [Member] | Exelon Wind Acquisition [Member]
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future Amortization Expense Year One			6,000,000
Future Amortization Expense Year Two			13,000,000
Future Amortization Expense Year Three			14,000,000
Future Amortization Expense Year Four			14,000,000
Future Amortization Expense Year Five			14,000,000
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross			224,000,000
Finite lived intangible assets accumulated amortization			(7,000,000)
Finite lived intangible assets net			217,000,000
John Deere Renewables [Abstract]
PPA's for projects in advanced stages of development			48,000,000
Constellation Energy Group Acquisition [Member]
CEG Acquisition [Abstract]
CEG Exchange Ratio			0.930
Projected Transaction Costs			144,000,000
Direct Merger Investment to State			250,000,000
Termination Costs from Acquirer to Acquiree			800,000,000
Termination Costs from Acquiree to Exelon			200,000,000
Exelon Wind Acquisition [Member]
John Deere Renewables [Abstract]
PPA's for projects in advanced stages of development			$ 48,000,000

[1] On September 30, 2010, Generation issued $900 million of senior notes, the proceeds of which were used to fund the acquisition.