Mergers, Acquisitions, and Dispositions - Narrative (Details) - USD ($) $ in Millions				3 Months Ended			12 Months Ended
	Apr. 04, 2018	Feb. 28, 2018	Mar. 31, 2017	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross				$ 0		$ 212
Bargain Purchase Gain				0		226
Fitzpatrick [Member]
Business Acquisition [Line Items]
Bargain Purchase Gain					$ 7	226	$ 233
Business Combination, Integration Related Costs						32
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		$ 87		0		212
Bargain Purchase Gain				$ 0		226
Exelon Generation Co L L C [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 62
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	226	233
Business Combination, Integration Related Costs						$ 32
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