Revenue from Contracts with Customers Change in Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Asset, Net, Current	$ 187	$ 187	$ 283
Contract with Customer, Liability, Current	25	27	35
Increases (decreases) as a result of changes in the estimate of the stage of completion, contract assets	26	50
Increases (decreases) as a result of changes in the estimate of the stage of completion, contract liabilities	0	0
Increases (decreases) as a result of additional cash received or due, contract assets	0	0
Increases (decreases) as a result of additional cash received or due, contract liabilities	21	179
Amounts reclassified to receivables, contract assets	(26)	(146)
Amounts recognized into revenues, contract liabilities	(23)	(187)
Exelon Generation Co L L C [Member]
Change in Contract with Customer, Asset and Liability [Line Items]
Contract with Customer, Asset, Net, Current	187	187	283
Contract with Customer, Liability, Current	39	42	$ 35
Increases (decreases) as a result of changes in the estimate of the stage of completion, contract assets	26	50
Increases (decreases) as a result of changes in the estimate of the stage of completion, contract liabilities	0	0
Increases (decreases) as a result of additional cash received or due, contract assets	0	0
Increases (decreases) as a result of additional cash received or due, contract liabilities	63	465
Amounts reclassified to receivables, contract assets	(26)	(146)
Amounts recognized into revenues, contract liabilities	$ (66)	$ (458)