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Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Exelon Generation Co L L C [Member] | Derivative [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ 464,000,000 [1],[2] $ 435,000,000 [3],[4]
Exelon Generation Co L L C [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Inputs [Abstract]    
Forward power price assets 8 11
Forward gas price assets 1.92 1.72
Exelon Generation Co L L C [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Inputs [Abstract]    
Forward power price assets 130 130
Forward gas price assets $ 9.87 $ 9.2
Exelon Generation Co L L C [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member]    
Fair Value Inputs [Abstract]    
Volatility percentage 13.00% 8.00%
Exelon Generation Co L L C [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member]    
Fair Value Inputs [Abstract]    
Volatility percentage 112.00% 173.00%
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral $ 3,000,000 [1],[2] $ (3,000,000) [3],[4]
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Inputs [Abstract]    
Forward power price assets 15 19
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Inputs [Abstract]    
Forward power price assets 67 79
Commonwealth Edison Co [Member] | Derivative [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ (282,000,000) $ (258,000,000)
Commonwealth Edison Co [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Inputs [Abstract]    
Forward heat rate 8.00% 8.00%
Derivatives Fair Value Footnotes [Abstract]    
Fair Value Inputs, Discount for Lack of Marketability 3.00% 3.00%
Fair Value Inputs Renewable Factor 88.00% 89.00%
Commonwealth Edison Co [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Inputs [Abstract]    
Forward heat rate 9.00% 9.00%
Derivatives Fair Value Footnotes [Abstract]    
Fair Value Inputs, Discount for Lack of Marketability 8.00% 8.00%
Fair Value Inputs Renewable Factor 121.00% 121.00%
[1] (a)The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.
[2] The fair values do not include cash collateral posted on level three positions of $98 million as of March 31, 2017.
[3] The fair values do not include cash collateral posted on level three positions of $61 million as of December 31, 2016.
[4] The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.