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, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	$ 172	$ 174	$ 187	$ 187	$ 174	$ 187
Consideration (received) due, contract assets	(26)	(19)	(18)	(26)
Revenues recognized, contract assets	13	17	27	26
Ending Balance - Contract Assets	159	172	196	187	159	196
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	29	33	25	27	33	27
Consideration received (due), contract liabilities	13	20	17	21
Revenues recognized, contract liabilities	(22)	(24)	(21)	(23)
Ending Balance - Contract Liabilities	20	29	21	25	20	21
Contract with Customer, Performance Obligation Satisfied in Previous Period	14		9		23	14
Exelon Generation Co L L C [Member]
Contract Asset [Roll Forward]
Beginning Balance - Contract Assets	172	174	187	187	174	187
Consideration (received) due, contract assets	(26)	(19)	(18)	(26)
Revenues recognized, contract assets	13	17	27	26
Ending Balance - Contract Assets	159	172	196	187	159	196
Contract Liabilities [Roll Forward]
Beginning Balance - Contract Liabilities	56	71	39	42	71	42
Consideration received (due), contract liabilities	34	55	52	63
Revenues recognized, contract liabilities	(63)	(70)	(65)	(66)
Ending Balance - Contract Liabilities	27	$ 56	26	$ 39	27	26
Contract with Customer, Performance Obligation Satisfied in Previous Period	$ 42		$ 24		$ 61	$ 28