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Mergers, Acquisitions, and Dispositions - Narrative (Details) - USD ($) $ in Millions					3 Months Ended			9 Months Ended		12 Months Ended
	Jul. 09, 2018	Apr. 04, 2018	Feb. 28, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross								$ 57	$ 208
Bargain Purchase Gain					$ 0	$ 7		0	233
Disposal Group, Including Discontinued Operation, Assets, Current					910			910		$ 0
Disposal Group, Including Discontinued Operation, Liabilities, Current					788			788		0
Asset Retirement Obligation, Revision of Estimate								(20)
Fitzpatrick [Member]
Business Acquisition [Line Items]
Business Combination, Integration Related Costs					0			0
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross								57	208
Bargain Purchase Gain					0	7		0	233
Disposal Group, Including Discontinued Operation, Assets, Current					910			910		0
Disposal Group, Including Discontinued Operation, Liabilities, Current					788			788		0
Asset Retirement Obligation, Revision of Estimate					84			0
Proceeds from Divestiture of Businesses			$ 87
Exelon Generation Co L L C [Member] \| FirstEnergy [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 140
Exelon Generation Co L L C [Member] \| Handley [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		$ 62
Exelon Generation Co L L C [Member] \| Fitzpatrick [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross										110
Business Combination, Consideration Transferred				$ 289						289
Business Combination, Consideration Transferred, Other				179						54
Bargain Purchase Gain						7	$ 226			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]
Payments to Acquire Businesses, Gross										$ 125
Business Combination, Consideration Transferred				$ 110