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Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details) - Derivative [Member] - Fair Value, Inputs, Level 3 [Member]
$ in Millions
12 Months Ended
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral $ (219) $ (301)
Minimum [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.03 0.03
Fair Value Inputs Forward Heat Rate 900.00% 800.00%
Renewable factor 92.00% 91.00%
Maximum [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.07 0.08
Fair Value Inputs Forward Heat Rate 1000.00% 900.00%
Renewable factor 120.00% 123.00%
Arithmetic Average [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.0477 0.0493
Fair Value Inputs Forward Heat Rate 913.00% 885.00%
Renewable factor 97.00% 99.00%
Commonwealth Edison Co [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral $ (219) $ (301)
Commonwealth Edison Co [Member] | Minimum [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.03 0.03
Fair Value Inputs Forward Heat Rate 900.00% 800.00%
Renewable factor 92.00% 91.00%
Commonwealth Edison Co [Member] | Maximum [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.07 0.08
Fair Value Inputs Forward Heat Rate 1000.00% 900.00%
Renewable factor 120.00% 123.00%
Commonwealth Edison Co [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative Asset (Liability) Net, Measurement Input 0.0477 0.0493
Fair Value Inputs Forward Heat Rate 913.00% 885.00%
Renewable factor 97.00% 99.00%