Nuclear Decommissioning (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	$ 10,000,000		$ 5,000,000
Nuclear Decommissioning Asset Retirement Obligations [Abstract]
Net adjustment to nuclear ARO	1,061,000,000		404,000,000
Resulting pre-tax income (loss) from nuclear ARO adjustment	(10,000,000)		(28,000,000)
Nuclear Decommissioning Trust Fund Investments [Abstract]
NDT fund investments	7,248,000,000		6,507,000,000
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79,000,000		65,000,000
Pledged assets for Zion Station decommissioning	614,000,000		734,000,000
Total payable to ZionSolutions	564,000,000	[1]	691,000,000	[1]
Current payable to ZionSolutions	132,000,000	[2]	128,000,000	[2]
Zion Station decommissioning costs withdrawn	192,000,000		143,000,000
ZionSolutions rent payable	1
EnergySolutions letter of credit	200,000,000
NRC Minimum Funding Requirements [Abstract]
Assumed annual after-tax return on NDT funds	2.00%
Assumed annual after-tax return on NDT funds for former PECO units	3.00%
Annual average accretion of the ARO	5.00%
Historical five-year annual average after-tax return on NDT funds	3.60%
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	386,000,000	[3],[4],[5]	(74,000,000)	[3],[4],[5]	294,000,000	[3],[4],[5]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	105,000,000	[5]	(4,000,000)	[5]	104,000,000	[5]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	73,000,000		48,000,000
Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	3,680,000,000	[6]	3,276,000,000
Accretion expense	231,000,000		209,000,000
Net increase (decrease) resulting from updates to estimated future cash flows	833,000,000		198,000,000
Costs incurred to decommission retired plants	(3,000,000)		(3,000,000)
ARO ending balance	4,741,000,000	[6]	3,680,000,000	[6]
Exelon Generation Co L L C [Member]
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	10,000,000		5,000,000
Reduction to operating and maintenance expense due to updates to estimated future cash flows	8,000,000
Nuclear Decommissioning Asset Retirement Obligations [Abstract]
Net adjustment to nuclear ARO	1,061,000,000		404,000,000
Resulting pre-tax income (loss) from nuclear ARO adjustment	(10,000,000)		(28,000,000)
Nuclear Decommissioning Trust Fund Investments [Abstract]
NDT fund investments	7,248,000,000		6,507,000,000
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79,000,000		65,000,000
Pledged assets for Zion Station decommissioning	614,000,000		734,000,000
Total payable to ZionSolutions	564,000,000	[1]	691,000,000	[1]
Current payable to ZionSolutions	132,000,000	[2]	128,000,000	[2]
Zion Station decommissioning costs withdrawn	192,000,000		143,000,000
ZionSolutions rent payable	1
EnergySolutions letter of credit	200,000,000
NRC Minimum Funding Requirements [Abstract]
Assumed annual after-tax return on NDT funds	2.00%
Assumed annual after-tax return on NDT funds for former PECO units	3.00%
Annual average accretion of the ARO	5.00%
Historical five-year annual average after-tax return on NDT funds	3.60%
NRC funding assurance parent guarantees	115,000,000
Unrealized Losses On Nuclear Decommissioning Trust Fund Investment [Line Items]
Net unrealized gains (losses) on decommissioning trust funds - Regulatory Agreement Units	386,000,000	[3],[4],[5]	(74,000,000)	[3],[4],[5]	294,000,000	[3],[4],[5]
Net unrealized gains (losses) on decommissioning trust funds - Non-Regulatory Agreement Units	105,000,000	[5]	(4,000,000)	[5]	104,000,000	[5]
Footnotes To Unrealized Gains Losses On NDT Funds [Abstract]
Gains on Zion Station Pledged Assets	73,000,000		48,000,000
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	3,680,000,000	[6]	3,276,000,000
Accretion expense	231,000,000		209,000,000
Net increase (decrease) resulting from updates to estimated future cash flows	833,000,000		198,000,000
Costs incurred to decommission retired plants	(3,000,000)		(3,000,000)
ARO ending balance	$ 4,741,000,000	[6]	$ 3,680,000,000	[6]

[1] Excludes a liability recorded within 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 Generation’s Consolidated Balance Sheets.

[3] Net unrealized gains (losses) 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.

[4] Excludes $73 million and $48 million of net unrealized gains (losses) related to the Zion Station pledged assets in 2012 and 2011. 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.

[5] Net unrealized gains (losses) related to Generation's NDT funds are included within other, net in Exelon's and Generation's Consolidated Statements of Operations and Comprehensive Income.

[6] Includes $10 million and $5 million as the current portion of the ARO at December 31, 2012 and 2011, respectively, which is included in other current liabilities on Exelon's and Generation's Consolidated Balance Sheets.