Early Plant Retirements Early Plant Retirements - Pretax Expense (Details) - Facility Closing [Member] - 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
Three Mile Island [Member]
Restructuring and Related Cost, Accelerated Depreciation	[1],[2]		$ 109	$ 106	$ 35		$ 250
Accelerated Nuclear Fuel Amortization	[2]		4	6	2		12
Operating and maintenance	[3]		1	5	71		77
Restructuring and Related Cost, Incurred Cost			114	117	108		339
TMI,OysterCreek [Member]
Restructuring and Related Cost, Incurred Cost		$ 174				$ 525
Exelon Generation Co L L C [Member] | Three Mile Island [Member]
Restructuring and Related Cost, Accelerated Depreciation	[1],[2]		109	106	35		250
Accelerated Nuclear Fuel Amortization	[2]		4	6	2		12
Operating and maintenance	[3]		1	5	71		77
Restructuring and Related Cost, Incurred Cost			$ 114	$ 117	$ 108		$ 339
Exelon Generation Co L L C [Member] | TMI,OysterCreek [Member]
Restructuring and Related Cost, Accelerated Depreciation	[4],[5]	152				441
Accelerated Nuclear Fuel Amortization	[4]	18				52
Operating and maintenance	[6]	4				32
Restructuring and Related Cost, Incurred Cost		$ 174				$ 525

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

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

[3] Primarily includes materials and supplies inventory reserve adjustments, employee related costs and CWIP impairments.

[4] Reflects incremental accelerated depreciation and amortization for TMI and Oyster Creek for the three and nine months ended September 30, 2018. The Oyster Creek year-to-date amounts are from February 2, 2018 through September 17, 2018.

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

[6] Primarily includes materials and supplies inventory reserve adjustments, employee related costs and CWIP impairments. It does not include remeasurement of the Oyster Creek ARO. Refer to Note 13 - Asset Retirement Obligations for additional detail.