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, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	$ 159	$ 172	$ 174	$ 196	$ 187	$ 187	$ 174	$ 187
Consideration (received) due, contract assets	(18)	(26)	(19)	(65)	(18)	(26)
Revenues recognized, contract assets	19	13	17	39	27	26
Ending Balance - Contract Assets	160	159	172	170	196	187	160	170
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	20	29	33	21	25	27	33	27
Consideration received (due), contract liabilities	154	13	20	27	17	21
Revenues recognized, contract liabilities	(25)	(22)	(24)	(22)	(21)	(23)
Ending Balance - Contract Liabilities	149	20	29	26	21	25	149	26
Contract with Customer, Performance Obligation Satisfied in Previous Period	2			3			25	17
Exelon Generation Co L L C [Member]
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	159	172	174	196	187	187	174	187
Consideration (received) due, contract assets	(18)	(26)	(19)	(65)	(18)	(26)
Revenues recognized, contract assets	19	13	17	39	27	26
Ending Balance - Contract Assets	160	159	172	170	196	187	160	170
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	27	56	71	26	39	42	71	42
Consideration received (due), contract liabilities	94	34	55	83	52	63
Revenues recognized, contract liabilities	(65)	(63)	(70)	(61)	(65)	(66)
Ending Balance - Contract Liabilities	56	27	56	48	26	39	56	48
Contract with Customer, Performance Obligation Satisfied in Previous Period	2			3			63	32
Pepco Holdings LLC [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0	0	0	0	0	0
Consideration received (due), contract liabilities	124	0	0	0	0	0
Revenues recognized, contract liabilities	(2)	0	0	0	0	0
Ending Balance - Contract Liabilities	122	0	0	0	0	0	122	0
Potomac Electric Power Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0	0	0	0	0	0
Consideration received (due), contract liabilities	98	0	0	0	0	0
Revenues recognized, contract liabilities	(2)	0	0	0	0	0
Ending Balance - Contract Liabilities	96	0	0	0	0	0	96	0
Delmarva Power and Light Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0	0	0	0	0	0
Consideration received (due), contract liabilities	13	0	0	0	0	0
Revenues recognized, contract liabilities	0	0	0	0	0	0
Ending Balance - Contract Liabilities	13	0	0	0	0	0	13	0
Atlantic City Electric Company [Member]
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	0	0	0	0	0	0	0	0
Consideration received (due), contract liabilities	13	0	0	0	0	0
Revenues recognized, contract liabilities	0	0	0	0	0	0
Ending Balance - Contract Liabilities	$ 13	$ 0	$ 0	$ 0	$ 0	$ 0	$ 13	$ 0