Nuclear Decommissioning - Rollforward of Nuclear Decommissioning ARO (Details) (Nuclear Decommissioning Asset Retirement Obligation [Member], Exelon Generation Co L L C [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear Decommissioning ARO, Beginning Balance	$ 6,961	[1]
Accretion expense	94
Nuclear Decommissioning ARO, Ending Balance	7,110	[1]
Current portion of ARO	8		9
Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Nuclear Decommissioning ARO, Ending Balance	$ 55	[2]

[1] Includes $8 million as the current portion of the ARO at March 31, 2015 and December 31, 2014, which is included in Other current liabilities on Exelon’s and Generation’s Consolidated Balance Sheets.

[2] 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.