Asset Retirement Obligation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Nuclear Decommissioning Asset Retirement Obligations [Abstract]
Increase in estimated cash flows associated with Zion Station decommissioning	$ 0
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79
Nuclear Decommissioning Trust Fund Investments [Abstract]
Shortfall of decommissioning funds with recourse	50
Percent of additional decommissioning shortfall with recourse	5.00%
NDT fund investments	7,559			7,248
NRC Minimum Funding Requirements [Abstract]
Additional NRC funding assurance parent guarantees				115
Nuclear Decommissioning Obligation [Line Items]
Pledged assets for Zion Station decommissioning	580			614
Total payable to ZionSolutions	531			564
Current payable to ZionSolutions	159			132
Zion Station decommissioning costs withdrawn	371			335
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	195	[1]		247	[1]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	64	[2],[3]		65	[2],[3]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	2		35
Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	4,741	[4]
Accretion expense	64
Net increase (decrease) resulting from updates to estimated future cash flows	0
Costs incurred to decommission retired plants	(1)
ARO ending balance	4,804	[4]
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	10			10
Exelon Generation Co L L C [Member]
Nuclear Decommissioning Asset Retirement Obligations [Abstract]
Increase in estimated cash flows associated with Zion Station decommissioning	0
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79
Nuclear Decommissioning Trust Fund Investments [Abstract]
Shortfall of decommissioning funds with recourse	50
Percent of additional decommissioning shortfall with recourse	5.00%
NDT fund investments	7,559			7,248
NRC Minimum Funding Requirements [Abstract]
Additional NRC funding assurance parent guarantees				115
Nuclear Decommissioning Obligation [Line Items]
Pledged assets for Zion Station decommissioning	580			614
Total payable to ZionSolutions	531			564
Current payable to ZionSolutions	159			132
Zion Station decommissioning costs withdrawn	371			335
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	195	[1]		247	[1]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	64	[2],[3]		65	[2],[3]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	2		35
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	4,741	[4]
Accretion expense	64
Net increase (decrease) resulting from updates to estimated future cash flows	0
Costs incurred to decommission retired plants	(1)
ARO ending balance	4,804	[4]
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	$ 10			$ 10

[1] Net unrealized gains related to Generation's NDT funds associated with Regulatory Agreement Units are included in regulatory liabilities on Exelon's Consolidated Balance Sheets and noncurrent payables to affiliates on Generation's Consolidated Balance Sheets.

[2] Net unrealized gains related to Generation's NDT funds associated with Non-Regulatory Agreement Units are included within Other, net in Exelon's and Generation's Consolidated Statements of Operations and Comprehensive Income.

[3] Excludes $2 and $35 million of net unrealized gains related to the Zion Station pledged assets for the three months ended March 31, 2013 and 2012, respectively. Net unrealized gains related to Zion Station pledged assets are included in the payable for Zion Station decommissioning on Exelon's and Generation's Consolidated Balance Sheets.

[4] (a)        Includes $10 million as the current portion of the ARO at March 31, 2013 and December 31, 2012, which is included in other current liabilities on Exelon's and Generation's Consolidated Balance Sheets.