Income Taxes - Recognition of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 73	$ 462	$ 463
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Exelon Generation Co L L C [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	39	429	408
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	33	32	31
Atlantic City Electric Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Increase in Unrecognized Tax Benefits is Reasonably Possible	14	$ 14	$ 14
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	14
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	14
Tax Year 2019 [Member] | State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 21