Fair Value of Financial Assets and Liabilities (Fair Value Inputs Assets Quantitative Information) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013
Derivatives Fair Value [Line Items]
Forward Power Basis	2.50
Forward Gas Basis	0.15
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded	9,000,000
Fair Value Inputs Level 3 [Member]
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded
Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	8.00%	[1]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Net	346,000,000	[2],[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	14,000,000	[3]
Forward gas price assets	3,430,000	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	88,000,000	[3]
Forward gas price assets	4,730,000	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	28.00%	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	177.00%	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member]
Derivatives Fair Value [Line Items]
Derivative Liability, Fair Value, Net	(11,000,000)	[2],[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	16,000,000	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Forward power price assets	88,000,000	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member] | Option Model Valuation Technique [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	14.00%	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member] | Option Model Valuation Technique [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Volatility percentage	34.00%	[3]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Net	85,000,000	[1],[2]
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current asset	85,000,000
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	7.00%	[1]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	8.00%	[1]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Derivative Liability, Fair Value, Net	(75,000,000)	[2]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	3.50%
Forward heat rate	8.50%	[4]
Renewable factor	85.00%
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	8.30%
Forward heat rate	9.50%	[4]
Renewable factor	128.00%
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Net	85,000,000	[1],[2]
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current liability	85,000,000
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	7.00%	[1]

[1] Includes current assets for Generation and current liabilities for ComEd of $85 million, related to the fair value of the five-year financial swap contract between Generation and ComEd, which eliminates in consolidation.

[2] The fair values do not include cash collateral held on level three positions which are immaterial as of March 31, 2013

[3] The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.

[4] Quoted forward natural gas rates are utilized to project the forward power curve for the delivery of energy at specified future dates. The natural gas curve is extrapolated beyond its observable period to the end of the contract’s delivery.