Income Taxes Income Taxes - Reasonably possible the total amount of unrecognized tax benefits could significantly increase or decrease within 12 months after the reporting date (Details) $ in Millions	Sep. 30, 2019 USD ($)
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 425
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	411
Exelon Generation Co L L C [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	411
Pepco Holdings LLC [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	14
Atlantic City Electric Company [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 14