Mergers, Acquisitions, and Dispositions - Narrative (Details) - USD ($) $ in Millions				3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 04, 2018	Feb. 28, 2018	Mar. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross								$ 57	$ 212
Bargain Purchase Gain				$ 0		$ 0		0	226
Fitzpatrick [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain							$ 226
Business Combination, Integration Related Costs				0		16		0	47
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 62	$ 87						57	212
Bargain Purchase Gain				0		0		0	226
Exelon Generation Co L L C [Member] | Fitzpatrick [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred			$ 289							$ 289
Business Combination, Consideration Transferred, Other			179
Bargain Purchase Gain					$ 7					233
Business Combination, Integration Related Costs				$ 0		$ 16		$ 0	$ 47
Exelon Generation Co L L C [Member] | Fitzpatrick [Member] | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred			$ 110							110
Business Combination, Consideration Transferred, Other										$ 125