Revenue from Contracts with Customers - Change in Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	$ 153	$ 144	$ 172	$ 174	$ 144	$ 174
Consideration (received) due, contract assets	(12)	(16)	(26)	(19)
Revenues recognized, contract assets	9	13	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination		12
Ending Balance - Contract Assets	150	153	159	172	150	159
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	147	151	29	33	151	33
Consideration received (due), contract liabilities	17	20	13	20
Revenues recognized, contract liabilities	(32)	(27)	(22)	(24)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		3
Ending Balance - Contract Liabilities	132	147	20	29	132	20
Contract with Customer, Performance Obligation Satisfied in Previous Period	17		14		34	23
Exelon Generation Co L L C [Member]
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	153	144	172	174	144	174
Consideration (received) due, contract assets	(12)	(16)	(26)	(19)
Revenues recognized, contract assets	9	13	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination		12
Ending Balance - Contract Assets	150	153	159	172	150	159
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	54	84	56	71	84	71
Consideration received (due), contract liabilities	39	31	34	55
Revenues recognized, contract liabilities	(68)	(64)	(63)	(70)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		3
Ending Balance - Contract Liabilities	25	54	27	56	25	27
Contract with Customer, Performance Obligation Satisfied in Previous Period	41		42		79	61
Pepco Holdings LLC [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	116	118	0	0	118	0
Consideration received (due), contract liabilities	0	0	0	0
Revenues recognized, contract liabilities	(3)	(2)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		0
Ending Balance - Contract Liabilities	113	116	0	0	113	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	3		0		5	0
Potomac Electric Power Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	92	94	0	0	94	0
Consideration received (due), contract liabilities	0	0	0	0
Revenues recognized, contract liabilities	(1)	(2)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		0
Ending Balance - Contract Liabilities	91	92	0	0	91	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	1		0		3	0
Delmarva Power and Light Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	12	12	0	0	12	0
Consideration received (due), contract liabilities	0	0	0	0
Revenues recognized, contract liabilities	(1)	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		0
Ending Balance - Contract Liabilities	11	12	0	0	11	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	1		0		1	0
Atlantic City Electric Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	12	12	0	0	12	0
Consideration received (due), contract liabilities	0	0	0	0
Revenues recognized, contract liabilities	(1)	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination		0
Ending Balance - Contract Liabilities	11	$ 12	0	$ 0	11	0
Contract with Customer, Performance Obligation Satisfied in Previous Period	$ 1		$ 0		$ 1	$ 0