Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2019
Income Taxes [Line Items]
Income Tax Credits and Adjustments		$ 108
Current Federal, State and Local, Tax Expense (Benefit)		97
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 411
Like kind exchange [Member]
Income Taxes [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible				$ 33
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Income Tax Credits and Adjustments		75
Current Federal, State and Local, Tax Expense (Benefit)		66
Decrease in Unrecognized Tax Benefits is Reasonably Possible	411
Commonwealth Edison Co [Member] | Like kind exchange [Member]
Income Taxes [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible				$ 2
Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Increase in Unrecognized Tax Benefits is Reasonably Possible	14	14	$ 14
State and Local Jurisdiction [Member]
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes	(66)	(23)	50
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes	26	$ (9)	$ 53
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]
Income Taxes [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	14
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	14
Tax Year 2019 [Member] | State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Income Taxes [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 21