Intangible Assets Intangible Assets - Summary of Amortization Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	$ (17)	$ (28)	$ (109)
Exelon Generation Co L L C [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	81	74	63
Commonwealth Edison Co [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	7	7	7
Pepco Holdings LLC [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible asset amortization expense	(115)	(119)	(188)
Unamortized Energy Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Power Contracts Emission Credits	30	21	14
Unamortized Energy Contracts [Member] | Exelon Generation Co L L C [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Power Contracts Emission Credits	30	21	14
Unamortized Energy Contracts [Member] | Commonwealth Edison Co [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Power Contracts Emission Credits	0	0	0
Unamortized Energy Contracts [Member] | Pepco Holdings LLC [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Power Contracts Emission Credits	$ 0	$ 0	$ 0