Early Nuclear Plant Retirements Early Nuclear Plant Retirements - Narrative (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net		$ 74,711	$ 74,202			$ 74,202
Facility Closing [Member] | Mystic Generating Station [Member]
Property, Plant and Equipment [Line Items]
Inventory, Raw Materials, Gross		26
Nuclear Fuel, Net of Amortization		18
Property, plant and equipment, net		896
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		4
Prepaid Expense	[1]	9
Asset Retirement Obligation		(5)
Accrued Liabilities	[1]	(2)
Facility Closing [Member] | Salem [Member]
Property, Plant and Equipment [Line Items]
Inventory, Raw Materials, Gross		45
Nuclear Fuel, Net of Amortization		102
Property, plant and equipment, net		618
Asset Retirement Obligation		(446)
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] | TMI,OysterCreek [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Incurred Cost		178
Exelon Generation Co L L C [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net		$ 24,714	$ 24,906			$ 24,906
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%

[1] (a)Reflects ending balances only as they relate to Mystic's Long-term Service Agreement.