Variable Interest Entities - Narrative (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018 USD ($) VIE	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) VIE	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) VIE	Apr. 01, 2014 USD ($)
Variable Interest Entity [Line Items]
Number Of Variable Interest Entities Consolidated | VIE		5		5		5
Number of Variable Interest Entities not consolidated by equity holders | VIE		7		7		7
Guarantor Obligations, Maximum Exposure, Undiscounted		$ 60		$ 60
Other Nonconsolidated Variable Interest Entity Ownership Interests	[1]	598		598		$ 618
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		15		15
Payment Guarantee [Member] | Constellation Energy Nuclear Group [Member]
Variable Interest Entity [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted							$ 245
Financial Guarantee [Member] | Constellation Energy Nuclear Group [Member]
Variable Interest Entity [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted							165
Exelon Generation Co L L C [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Financial or Other Support, Amount				$ 34
Related Party Purchase Of Nuclear Output By Third Party Percentage				50.01%
Other Nonconsolidated Variable Interest Entity Ownership Interests		15		$ 15
Exelon Generation Co L L C [Member] | Constellation Energy Nuclear Group [Member]
Variable Interest Entity [Line Items]
Receivables from affiliates		194		194			$ 400
Exelon Generation Co L L C [Member] | Variable Interest Entity, Primary Beneficiary [Member] | Equity Method Investment Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Guarantor Obligations, Current Carrying Value		637		637
Atlantic City Electric Company [Member]
Variable Interest Entity [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted		17		17
Atlantic City Electric Company [Member] | ATF [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Financial or Other Support, Amount		$ 9	$ 11	$ 23	$ 39

[1] These items represent amounts on the unconsolidated VIE balance sheets, not on Exelon’s or Generation’s Consolidated Balance Sheets. These items are included to provide information regarding the relative size of the unconsolidated VIEs.