Income Taxes - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Jan. 01, 2021	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Additional Narrative Information [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)			$ (33)
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 425			$ 425
Unrecognized tax benefits that if recognized would affect the effective tax rate		$ 411			$ 411
Effective income tax rate		22.40%		10.80%	23.60%	9.50%
Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 411			$ 411
Unrecognized tax benefits that if recognized would affect the effective tax rate		$ 411			$ 411
Effective income tax rate		38.60%		11.00%	35.20%	7.80%
Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)			$ (2)
Effective income tax rate		19.80%		20.80%	19.80%	21.10%
PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate		11.30%		(1.10%)	12.10%	(1.40%)
Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate		19.60%		20.30%	18.60%	18.60%
Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 14			$ 14
Effective income tax rate		5.40%		15.20%	4.70%	13.90%
Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate		3.00%		11.50%	3.30%	9.60%
Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate		11.80%		16.10%	11.70%	17.40%
Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 14			$ 14
Effective income tax rate		0.00%		20.00%	4.00%	16.70%
ILLINOIS
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate					9.50%
Subsequent Event [Member] | ILLINOIS
Income Tax Additional Narrative Information [Line Items]
Effective income tax rate	10.49%