Impairment of Long-lived Assets Impairment of Long-Lived Assets - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended					9 Months Ended
	Mar. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2015
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Tangible Asset Impairment Charges					$ 36
Property, plant and equipment, net								$ 76,707		$ 74,202
Gain (Loss) on Contract Termination						$ 4
Finite-Lived Intangible Assets, Gross								1,267		1,228
Estimated residual value of leased assets	$ 356					356
Proceeds From Lease Termination						$ 360
Wind Assets [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Property, plant and equipment, net					60
Property, Plant, and Equipment, Fair Value Disclosure					$ 24
Pepco Holdings LLC [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment of Long-Lived Assets to be Disposed of			$ 25
Property, plant and equipment, net								13,446		12,498
Finite-Lived Intangible Assets, Gross											$ 25
Exelon Generation Co L L C [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Property, plant and equipment, net								$ 23,981		$ 24,906
Constellation Energy Group LLC [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Tangible Asset Impairment Charges	$ 119			$ 15
Electricity Generation Plant, Non-Nuclear [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impaired Assets to be Disposed of by Method Other than Sale, Carrying Value of Asset									$ 41
Impairment of Long-Lived Assets to be Disposed of		$ 41	$ 460				$ 460