Revenue from Contracts with Customers - Contract Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	$ 33	$ 27	$ 35
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	219	94	179
Contract with Customer, Liability, Revenue Recognized	(98)	(88)	(187)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	(3)
Ending Balance - Contract Liabilities	151	33	27
Exelon Generation Co L L C [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	71	42	35
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	282	287	465
Contract with Customer, Liability, Revenue Recognized	(266)	(258)	(458)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	(3)
Ending Balance - Contract Liabilities	84	71	42
Pepco Holdings LLC [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	122	0	0
Contract with Customer, Liability, Revenue Recognized	(4)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	118	0	0
Potomac Electric Power Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	98	0	0
Contract with Customer, Liability, Revenue Recognized	(4)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	94	0	0
Delmarva Power & Light Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	12	0	0
Contract with Customer, Liability, Revenue Recognized	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	12	0	0
Atlantic City Electric Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	12	0	0
Contract with Customer, Liability, Revenue Recognized	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	$ 12	$ 0	$ 0