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Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Dec. 31, 2020
USD ($)
Fair Value, Inputs, Level 3 [Member]    
Derivatives Fair Value Footnotes [Abstract]    
Cash Collateral Posted $ (590,000,000) $ 162,000,000
Fair Value, Inputs, Level 3 [Member] | Economic Hedges [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral (1,257,000,000) 245,000,000
Fair Value, Inputs, Level 3 [Member] | Derivative [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral $ (214,000,000) $ (301,000,000)
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 9.77 2.25
Forward gas price assets 1.76 1.57
Derivative Asset (Liability) Net, Measurement Input 0.03 0.03
Forward heat rate 900.00% 800.00%
Fair Value Inputs Renewable Factor 95.00% 91.00%
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 301 163
Forward gas price assets 23.00 7.88
Derivative Asset (Liability) Net, Measurement Input 0.07 0.08
Forward heat rate 1000.00% 900.00%
Fair Value Inputs Renewable Factor 122.00% 123.00%
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 55 30
Forward gas price assets 4.16 2.59
Derivative Asset (Liability) Net, Measurement Input 0.0477 0.0493
Forward heat rate 913.00% 885.00%
Fair Value Inputs Renewable Factor 100.00% 99.00%
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.35 0.11
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 1.97 2.37
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.49 0.32
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ (14,000,000) $ 23,000,000
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 16 10
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 156 106
Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 53 27
Exelon Generation Co L L C [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral $ 2,554,000,000 $ 297,000,000
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]    
Derivatives Fair Value Footnotes [Abstract]    
Cash Collateral Posted (590,000,000) 162,000,000
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Economic Hedges [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral $ (1,257,000,000) $ 245,000,000
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 9.77 2.25
Forward gas price assets 1.76 1.57
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 301 163
Forward gas price assets 23.00 7.88
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 55 30
Forward gas price assets 4.16 2.59
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.35 0.11
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 1.97 2.37
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Option Model Valuation Technique [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.49 0.32
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 $ (14,000,000) $ 23,000,000
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 16 10
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 156 106
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Proprietary Trading [Member] | Discounted Cash Flow [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Forward power price assets 53 27
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 $ (214,000,000) $ (301,000,000)
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.03 0.03
Forward heat rate 900.00% 800.00%
Fair Value Inputs Renewable Factor 95.00% 91.00%
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.07 0.08
Forward heat rate 1000.00% 900.00%
Fair Value Inputs Renewable Factor 122.00% 123.00%
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | Discounted Cash Flow [Member] | Arithmetic Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Abstract]    
Derivative Asset (Liability) Net, Measurement Input 0.0477 0.0493
Forward heat rate 913.00% 885.00%
Fair Value Inputs Renewable Factor 100.00% 99.00%