Asset Retirement Obligation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 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	81				81
Nuclear Decommissioning Trust Fund Investments [Abstract]
Shortfall of decommissioning funds with recourse	50				50
Percent of additional decommissioning shortfall with recourse	5.00%				5.00%
NDT fund investments	7,776				7,776				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	486				486				614
Total payable to ZionSolutions	443	[1]			443	[1]			564	[1]
Current payable to ZionSolutions	104	[2]			104	[2]			132	[2]
Zion Station decommissioning costs withdrawn					458	[3]			335	[3]
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	103	[4]	202	[4]	196	[4]	352	[4]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	46	[5],[6]	71	[5],[6]	70	[5],[6]	101	[5],[6]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	9		22		5		60
Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance					4,741	[7]
Accretion expense					194
Costs incurred to decommission retired plants					(2)
ARO ending balance	4,792	[7]			4,792	[7]
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	10				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	81				81
Nuclear Decommissioning Trust Fund Investments [Abstract]
Shortfall of decommissioning funds with recourse	50				50
Percent of additional decommissioning shortfall with recourse	5.00%				5.00%
NDT fund investments	7,776				7,776				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	486				486				614
Total payable to ZionSolutions	443	[1]			443	[1]			564	[1]
Current payable to ZionSolutions	104	[2]			104	[2]			132	[2]
Zion Station decommissioning costs withdrawn					458	[3]			335	[3]
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	103	[4]	202	[4]	196	[4]	352	[4]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	46	[5],[6]	71	[5],[6]	70	[5],[6]	101	[5],[6]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	9		22		5		60
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	[7]
Accretion expense					194
Costs incurred to decommission retired plants					(2)
ARO ending balance	4,792	[7]			4,792	[7]
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	$ 10				$ 10				$ 10

[1] Excludes a liability recorded within Exelon’s and Generation’s Consolidated Balance Sheets related to the tax obligation on the unrealized activity associated with the Zion Station NDT Funds. The NDT Funds will be utilized to satisfy the tax obligations as gains and losses are realized.

[2] Included in Other current liabilities within Exelon’s and Generation’s Consolidated Balance Sheets.

[3] Cumulative withdrawals since September 1, 2010.

[4] 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.

[5] 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.

[6] Excludes $9 million of net unrealized losses and $22 million of net unrealized gains related to the Zion Station pledged assets for the three months ended September 30, 2013 and 2012, respectively, and $5 million of net unrealized losses and $60 million of net unrealized gains related to the Zion Station pledged assets for the nine months ended September 30, 2013 and 2012, respectively. Net unrealized gains (losses) related to Zion Station pledged assets are included in the Payable for Zion Station decommissioning on Exelon's and Generation's Consolidated Balance Sheets.

[7] Includes $10 million as the current portion of the ARO at September 30, 2013 and December 31, 2012, respectively, which is included in Other current liabilities on Exelon's and Generation's Consolidated Balance Sheets.