Asset Retirement Obligation (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
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)
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79,000,000		65,000,000
ZionSolutions rent payable	1
EnergySolutions letter of credit	200,000,000
Nuclear Decommissioning Trust Fund Investments [Abstract]
Current annual decommissioning costs collected	24,000,000
Shortfall of decommissioning funds with recourse	50,000,000
Percent of additional decommissioning shortfall with recourse	5.00%
NDT fund investments	7,248,000,000		6,507,000,000
Percent of NDT funds invested in equity	47.00%		48.00%
Percent of NDT funds invested in fixed income securities	53.00%		52.00%
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%
Number of years used in present value measurement	30 years
Historical five-year annual average after-tax return on NDT funds	3.60%
Footnotes To Asset Retirement Obligation [Abstract]
Current Portion of ARO	10,000,000		5,000,000
Nuclear Decommissioning Obligation [Line Items]
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Zion Station decommissioning costs withdrawn	192,000,000		143,000,000
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]
Nonnuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	209,000,000		223,000,000
Development projects	47,000,000		7,000,000
Accretion expense	(13,000,000)	[7]	(9,000,000)	[7]
Net increase (decrease) resulting from updates to estimated future cash flows	27,000,000	[8]	(24,000,000)	[8]
Merger with Constellation	58,000,000	[9]
Payments	11,000,000		6,000,000
ARO ending balance	343,000,000		209,000,000
Minimum [Member]
NRC Minimum Funding Requirements [Abstract]
Years after cessation of plant operations	10 years
Estimated annual after-tax return on NDT funds	5.30%
Maximum [Member]
NRC Minimum Funding Requirements [Abstract]
Years after cessation of plant operations	70 years
Estimated annual after-tax return on NDT funds	6.20%
Exelon Generation Co L L C [Member]
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)
Zion Station Decommissioning [Abstract]
Zion Station spent nuclear fuel obligation	79,000,000		65,000,000
ZionSolutions rent payable	1
EnergySolutions letter of credit	200,000,000
Nuclear Decommissioning Trust Fund Investments [Abstract]
Current annual decommissioning costs collected	24,000,000
Shortfall of decommissioning funds with recourse	50,000,000
Percent of additional decommissioning shortfall with recourse	5.00%
NDT fund investments	7,248,000,000		6,507,000,000
Percent of NDT funds invested in equity	47.00%		48.00%
Percent of NDT funds invested in fixed income securities	53.00%		52.00%
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%
Number of years used in present value measurement	30 years
Historical five-year annual average after-tax return on NDT funds	3.60%
NRC funding assurance parent guarantees	115,000,000
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 Obligation [Line Items]
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
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]
Exelon Generation Co L L C [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	92,000,000		86,000,000
Development projects	47,000,000		7,000,000
Accretion expense	(8,000,000)	[7]	(5,000,000)	[7]
Net increase (decrease) resulting from updates to estimated future cash flows	18,000,000	[8]	(3,000,000)	[8]
Merger with Constellation	50,000,000	[9]
Payments	8,000,000		3,000,000
ARO ending balance	207,000,000		92,000,000
Exelon Generation Co L L C [Member] | Minimum [Member]
NRC Minimum Funding Requirements [Abstract]
Years after cessation of plant operations	10 years
Estimated annual after-tax return on NDT funds	5.30%
Exelon Generation Co L L C [Member] | Maximum [Member]
NRC Minimum Funding Requirements [Abstract]
Years after cessation of plant operations	70 years
Estimated annual after-tax return on NDT funds	6.20%
Commonwealth Edison Co [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	89,000,000		105,000,000
Accretion expense	(4,000,000)	[7]	(3,000,000)	[7]
Net increase (decrease) resulting from updates to estimated future cash flows	8,000,000	[8]	(17,000,000)	[8]
Payments	2,000,000		2,000,000
ARO ending balance	99,000,000		89,000,000
PECO Energy Co [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	28,000,000		32,000,000
Accretion expense	(1,000,000)	[7]	(1,000,000)	[7]
Net increase (decrease) resulting from updates to estimated future cash flows	1,000,000	[8]	(4,000,000)	[8]
Payments	1,000,000		1,000,000
ARO ending balance	29,000,000		28,000,000
Footnotes To Asset Retirement Obligation [Abstract]
Reduction to operating and maintenance expense due to updates to estimated future cash flows	3,000,000
Baltimore Gas and Electric Company [Member]
Footnotes To Asset Retirement Obligation [Abstract]
BGE ARO costs incurred prior to merger	8,000,000
Baltimore Gas and Electric Company [Member] | Nonnuclear Decommissioning Asset Retirement Obligation [Member]
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
ARO beginning balance	1,000,000
Net increase (decrease) resulting from updates to estimated future cash flows	7,000,000	[8]
ARO ending balance	$ 8,000,000				$ 1,000,000

[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.

[7] For ComEd and PECO, the majority of the accretion is recorded as an increase to a regulatory asset due to the associated regulatory treatment.

[8] During the year ended December 31, 2011, PECO recorded a reduction in operating and maintenance expense of $3 million. Generation and ComEd did not record any reductions in operating and maintenance expense for the year ended December 31, 2011. During the year ended December 31, 2012, Generation recorded a reduction in operating and maintenance expense of $8 million. ComEd and PECO did not record any adjustments in operating and maintenance expense for the year ended December 31, 2012.

[9] Exelon’s ARO includes $8 million of BGE costs incurred prior to the closing of Exelon’s merger with Constellation. Refer to Note 4 – Merger and Acquisitions for additional information.