Fair Value of Financial Assets and Liabilities (Fair Value Inputs Assets Quantitative Information) (Details) (USD $)									9 Months Ended										9 Months Ended						9 Months Ended						9 Months Ended						9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Derivative [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Income Approach Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Income Approach Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Option Model Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Option Model Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Proprietary Trading [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Proprietary Trading [Member] Income Approach Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Proprietary Trading [Member] Income Approach Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Proprietary Trading [Member] Option Model Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Proprietary Trading [Member] Option Model Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member] Income Approach Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Exelon Generation Co L L C [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member] Income Approach Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Derivative [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Income Approach Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Derivative [Member] Income Approach Valuation Technique [Member] Maximum [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member] Income Approach Valuation Technique [Member] Minimum [Member]		Sep. 30, 2012 Commonwealth Edison Co [Member] Fair Value Inputs Level 3 [Member] Business Intersegment Transactions [Member] Income Approach Valuation Technique [Member] Maximum [Member]
Derivatives Fair Value [Line Items]
Derivative Asset, Fair Value, Net			$ 386,000,000	[1],[2]																			$ 352,000,000	[1],[3]
Derivative Liability, Fair Value, Net													(83,000,000)	[1],[2]															(70,000,000)	[1]					352,000,000	[1],[3]
Forward Power Basis	4.00
Forward Gas Basis	0.25
Fair Value Inputs [Abstract]
Forward power price assets					8,000,000	[2]	76,000,000	[2]							13,000,000	[2]	108,000,000	[2]
Forward gas price assets					3,040,000	[2]	6,350,000	[2]
Volatility percentage									26.00%	[2]	120.00%	[2]							22.00%	[2]	67.00%	[2]
Marketability Reserve																									7.30%	[3]	8.70%	[3]			3.50%		8.30%				7.30%	[3]	8.70%	[3]
Forward heat rate																															8.5	[4]	9.5	[4]
Renewable factor																															80.00%		126.00%
Derivatives Fair Value Footnotes [Abstract]
Fair value swap contract current asset																							352,000,000
Fair value swap contract current liability		503,000,000																																	352,000,000
Cash collateral excluded	$ 9,000,000

[1] The fair values below do not include cash collateral held on level three positions of $9 million as of September 30, 2012

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

[3] Includes current assets for Generation and current liabilities for ComEd of $352 million, related to the fair value of the five-year financial swap contract between Generation and ComEd, which eliminates in consolidation.

[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.