Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash Collateral Posted	$ 162,000,000	$ 214,000,000
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets, fair value	$ 245,000,000	$ 558,000,000
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	2.25	9
Forward gas price assets	1.57	0.83
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.11	0.08
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	163	180
Forward gas price assets	7.88	10.72
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	2.37	2.36
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	30	29
Forward gas price assets	2.59	2.55
Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.32	0.70
Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets, fair value	$ 23,000,000	$ 45,000,000
Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	10	25
Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	106	180
Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	27	33
Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ (301,000,000)	$ (301,000,000)
Derivative [Member] | Fair Value, Inputs, Level 3 [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	800.00%	900.00%
Renewable factor	91.00%	91.00%
Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.08	0.07
Fair Value Inputs Forward Heat Rate	900.00%	1000.00%
Renewable factor	123.00%	123.00%
Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.0493	0.0495
Fair Value Inputs Forward Heat Rate	885.00%	968.00%
Renewable factor	99.00%	99.00%
Exelon Generation Co L L C [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ (297,000,000)	$ (307,000,000)
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash Collateral Posted	162,000,000	214,000,000
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets, fair value	$ 245,000,000	$ 558,000,000
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	2.25	9
Forward gas price assets	1.57	0.83
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.11	0.08
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	163	180
Forward gas price assets	7.88	10.72
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	2.37	2.36
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	30	29
Forward gas price assets	2.59	2.55
Exelon Generation Co L L C [Member] | Economic Hedges [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Option Model Valuation Technique [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.32	0.70
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets, fair value	$ 23,000,000	$ 45,000,000
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	10	25
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	106	180
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Forward power price assets	27	33
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ (301,000,000)	$ (301,000,000)
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [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	800.00%	900.00%
Renewable factor	91.00%	91.00%
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.08	0.07
Fair Value Inputs Forward Heat Rate	900.00%	1000.00%
Renewable factor	123.00%	123.00%
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | Arithmetic Average [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative Asset (Liability) Net, Measurement Input	0.0493	0.0495
Fair Value Inputs Forward Heat Rate	885.00%	968.00%
Renewable factor	99.00%	99.00%