Revenue from Contracts with Customers - Change in Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	$ 150	$ 153	$ 144	$ 159	$ 172	$ 174	$ 144	$ 174
Consideration (received) due, contract assets	(15)	(12)	(16)	(18)	(26)	(19)
Revenues recognized, contract assets	14	9	13	19	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination			12
Ending Balance - Contract Assets	149	150	153	160	159	172	149	160
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	132	147	151	20	29	33	151	33
Consideration received (due), contract liabilities	31	17	20	154	13	20
Revenues recognized, contract liabilities	(26)	(32)	(27)	(25)	(22)	(24)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			3
Ending Balance - Contract Liabilities	137	132	147	149	20	29	137	149
Contract with Customer, Performance Obligation Satisfied in Previous Period	4			2			38	25
Exelon Generation Co L L C [Member]
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	150	153	144	159	172	174	144	174
Consideration (received) due, contract assets	(15)	(12)	(16)	(18)	(26)	(19)
Revenues recognized, contract assets	14	9	13	19	13	17
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination			12
Ending Balance - Contract Assets	149	150	153	160	159	172	149	160
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	25	54	84	27	56	71	84	71
Consideration received (due), contract liabilities	93	39	31	94	34	55
Revenues recognized, contract liabilities	(65)	(68)	(64)	(65)	(63)	(70)
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			3
Ending Balance - Contract Liabilities	53	25	54	56	27	56	53	56
Contract with Customer, Performance Obligation Satisfied in Previous Period	2			2			81	63
Pepco Holdings LLC [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	113	116	118	0	0	0	118	0
Consideration received (due), contract liabilities	0	0	0	124	0	0
Revenues recognized, contract liabilities	(2)	(3)	(2)	(2)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			0
Ending Balance - Contract Liabilities	111	113	116	122	0	0	111	122
Contract with Customer, Performance Obligation Satisfied in Previous Period	2			0			7	0
Potomac Electric Power Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	91	92	94	0	0	0	94	0
Consideration received (due), contract liabilities	0	0	0	98	0	0
Revenues recognized, contract liabilities	(2)	(1)	(2)	(2)	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			0
Ending Balance - Contract Liabilities	89	91	92	96	0	0	89	96
Contract with Customer, Performance Obligation Satisfied in Previous Period	2			0			5	0
Delmarva Power and Light Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	11	12	12	0	0	0	12	0
Consideration received (due), contract liabilities	0	0	0	13	0	0
Revenues recognized, contract liabilities	0	(1)	0	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			0
Ending Balance - Contract Liabilities	11	11	12	13	0	0	11	13
Contract with Customer, Performance Obligation Satisfied in Previous Period	0			0			1	0
Atlantic City Electric Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	11	12	12	0	0	0	12	0
Consideration received (due), contract liabilities	0	0	0	13	0	0
Revenues recognized, contract liabilities	0	(1)	0	0	0	0
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			0
Ending Balance - Contract Liabilities	11	$ 11	$ 12	13	$ 0	$ 0	11	13
Contract with Customer, Performance Obligation Satisfied in Previous Period	$ 0			$ 0			$ 1	$ 0