Nuclear Decommissioning - Rollforward of Nuclear Decommissioning ARO (Details) (Exelon Generation Co L L C [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Period Increase (Decrease)	$ 1,800
Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear Decommissioning ARO, Beginning Balance	4,855	[1]
Accretion expense	243
Asset Retirement Obligation, Period Increase (Decrease)	(125)
Costs incurred to decommission retired plants	(5)
Nuclear Decommissioning ARO, Ending Balance	6,652	[1]
Current portion of ARO	9		9
Nuclear Decommissioning Asset Retirement Obligation [Member] | Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear Decommissioning ARO, Ending Balance	$ 1,684	[2]

[1] Includes $9 million as the current portion of the ARO at September 30, 2014 and December 31, 2013 which is included in Other current liabilities on Exelon’s and Generation’s Consolidated Balance Sheets.

[2] Includes the fair value of the CENG ARO liability as of April 1, 2014, the date of consolidation. See Note 6 — Investment in Constellation Energy Nuclear Group, LLC for additional information.