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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 ($)
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Exelon Generation Co L L C [Member] | Derivative [Member]      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ 788,000,000 [1],[2] $ 857,000,000 [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 6 11  
Forward gas price assets 1.24 1.18  
Exelon Generation Co L L C [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]      
Fair Value Inputs [Abstract]      
Forward power price assets 130 88  
Forward gas price assets $ 9.53 $ 8.95  
Exelon Generation Co L L C [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member]      
Fair Value Inputs [Abstract]      
Volatility percentage 5.00% 5.00%  
Exelon Generation Co L L C [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member]      
Fair Value Inputs [Abstract]      
Volatility percentage 115.00% 152.00%  
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] $ (7,000,000) [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 13  
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member]      
Fair Value Inputs [Abstract]      
Forward power price assets 68 78  
Commonwealth Edison Co [Member] | Derivative [Member]      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ (244,000,000) $ (247,000,000)  
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% 3.50%  
Fair Value Inputs Renewable Factor 86.00% 87.00%  
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% 7.00%  
Fair Value Inputs Renewable Factor 121.00% 128.00%  
[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 $150 million as of September 30, 2016.
[3] The fair values do not include cash collateral posted on level three positions of $201 million as of December 31, 2015.
[4] The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.