Early Nuclear Plant Retirements Early Nuclear Plant Retirements - Narrative (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net		$ 75,284	$ 74,202			$ 75,284	$ 74,202
Facility Closing [Member] | Mystic Generating Station [Member]
Property, Plant and Equipment [Line Items]
Inventory, Raw Materials, Gross		26				26
Nuclear Fuel, Net of Amortization		19				19
Property, plant and equipment, net		887				887
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		3				3
Prepaid Expense	[1]	11				11
Asset Retirement Obligation		(5)				(5)
Accrued Liabilities	[1]	(2)				(2)
Facility Closing [Member] | Salem [Member]
Property, Plant and Equipment [Line Items]
Inventory, Raw Materials, Gross		45				45
Nuclear Fuel, Net of Amortization		94				94
Property, plant and equipment, net		611				611
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		28				28
Asset Retirement Obligation		(451)				(451)
Facility Closing [Member] | Three Mile Island [Member]
Property, Plant and Equipment [Line Items]
Restructuring and Related Cost, Accelerated Depreciation	[2],[3]		109	$ 106	$ 35		250
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, Accelerated Depreciation	[4],[5]	152				289
Restructuring and Related Cost, Incurred Cost		173				351
Exelon Generation Co L L C [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net		$ 24,479	$ 24,906			$ 24,479	$ 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%				42.59%

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

[2] Reflects incremental accelerated depreciation of plant assets, including any ARC.

[3] Reflects incremental charges for TMI including incremental accelerated depreciation and amortization from May 30, 2017 through December 31, 2017.

[4] Reflects incremental accelerated depreciation and amortization for TMI and Oyster Creek for the three and six months ended June 30, 2018. The Oyster Creek year-to-date amounts are from February 2, 2018 through June 30, 2018.

[5] Reflects incremental accelerated depreciation of plant assets, including any ARC.