Early Plant Retirements Early Plant Retirements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Nuclear Plant [Member] | Exelon Generation Co L L C [Member]
Property, Plant and Equipment [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	42.59%				42.59%
Facility Closing [Member] | TMI,OysterCreek [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 174				$ 525
Facility Closing [Member] | TMI,OysterCreek [Member] | Exelon Generation Co L L C [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 174				$ 525
Facility Closing [Member] | Three Mile Island [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Incurred Cost		$ 114	$ 117	$ 108		$ 339
Facility Closing [Member] | Three Mile Island [Member] | Exelon Generation Co L L C [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Incurred Cost		$ 114	$ 117	$ 108		$ 339