Revenue from Contracts with Customers - Contract Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract Assets [Roll Forward]
Beginning Balance - Contract Assets	$ 174	$ 187
Amounts reclassified to receivables, contract assets	(86)	(143)
Revenues recognized, contract assets	68	130
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination	(12)
Ending Balance - Contract Assets	144	174
Exelon Generation Co L L C [Member]
Contract Assets [Roll Forward]
Beginning Balance - Contract Assets	174	187
Amounts reclassified to receivables, contract assets	(86)	(143)
Revenues recognized, contract assets	68	130
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination	(12)
Ending Balance - Contract Assets	$ 144	$ 174