Revenue from Contracts with Customers - Change in Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	$ 144	$ 174
Consideration (received) due, contract assets	(16)	(19)
Revenues recognized, contract assets	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination	12
Ending Balance - Contract Assets	153	172
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	151	33
Consideration received (due), contract liabilities	20	20
Revenues recognized, contract liabilities	(27)	(24)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	3
Ending Balance - Contract Liabilities	147	29
Contract with Customer, Performance Obligation Satisfied in Previous Period	17	9
Exelon Generation Co L L C [Member]
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	144	174
Consideration (received) due, contract assets	(16)	(19)
Revenues recognized, contract assets	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination	12
Ending Balance - Contract Assets	153	172
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	84	71
Consideration received (due), contract liabilities	31	55
Revenues recognized, contract liabilities	(64)	(70)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	3
Ending Balance - Contract Liabilities	54	56
Contract with Customer, Performance Obligation Satisfied in Previous Period	39	19
Pepco Holdings LLC [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	118	0
Consideration received (due), contract liabilities	0	0
Revenues recognized, contract liabilities	(2)	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	116	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	2	0
Potomac Electric Power Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	94	0
Consideration received (due), contract liabilities	0	0
Revenues recognized, contract liabilities	(2)	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	92	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	2	0
Delmarva Power and Light Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	12	0
Consideration received (due), contract liabilities	0	0
Revenues recognized, contract liabilities	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	12	0
Atlantic City Electric Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	12	0
Consideration received (due), contract liabilities	0	0
Revenues recognized, contract liabilities	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination	0
Ending Balance - Contract Liabilities	$ 12	$ 0