Fair Value of Financial Assets and Liabilities (Fair Value Inputs Assets Quantitative Information) (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Forward Power Basis	$ 1.96
Forward Gas Basis	0.18
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded	20,000,000
Fair Value Inputs Level 3 [Member]
Derivatives Fair Value Footnotes [Abstract]
Cash collateral excluded			33,000,000
Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			226,000,000	[1]
Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve			8.00%	[2]
Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve			9.00%	[2]
Exelon Generation Co L L C [Member]
Derivatives Fair Value Footnotes [Abstract]
Outstanding Commitments to invest	192,000,000
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	462,000,000	[3]	473,000,000	[1]
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current asset			226,000,000
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	15,000,000		14,000,000	[4]
Forward gas price assets	3.51		3.26	[4]
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	103,000,000		79,000,000	[4]
Forward gas price assets	5.97		6.27	[4]
Renewable factor			123.00%
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	27.00%		28.00%	[4]
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	107.00%		132.00%	[4]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Proprietary Trading [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			6,000,000	[1]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(19,000,000)	[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	(14,000,000)		(15,000,000)	[4]
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	(103,000,000)		(106,000,000)	[4]
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%		16.00%	[4]
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	28.00%		48.00%	[4]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member]
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current asset	0
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member]
Derivatives Fair Value [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	122,000,000	[3]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral			67,000,000	[1]
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%		3.50%
Forward heat rate	(8.00%)		(8.00%)	[5]
Renewable factor	84.00%		81.00%
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Marketability Reserve	8.00%		8.30%
Forward heat rate	(9.00%)		(9.50%)	[5]
Renewable factor	130.00%
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Business Intersegment Transactions [Member]
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current liability	$ 0

[1] The fair values do not include cash collateral held on level three positions of $33 million as of December 31, 2012.

[2] Includes current assets for Generation and current liabilities for ComEd of $226 million, related to the fair value of the five-year financial swap contract between Generation and ComEd, which eliminates in consolidation.

[3] The fair values do not include cash collateral held on level three positions of $20 million as of September 30, 2013.

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

[5] 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.