Impairment of Long-lived Assets Impairment of Long-Lived Assets - Narrative (Details) - USD ($) $ in Millions		1 Months Ended		3 Months Ended			12 Months Ended
		Sep. 30, 2017	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment charges					$ 36
Property, plant and equipment, net							$ 74,202	$ 71,555
Gain (Loss) on Contract Termination						$ 4
Interest costs incurred	[1]						1,658	1,678	$ 1,170
Finite-Lived Intangible Assets, Gross							1,228	1,244
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
Assets held for sale					$ 24
Pepco Holdings LLC [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Finite-Lived Intangible Assets, Gross										$ 25
PHI PEPCO [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment of Long-Lived Assets to be Disposed of				$ 25
Exelon Generation Co L L C [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Property, plant and equipment, net							24,906	25,585
Assets held for sale							0	17
Interest costs incurred	[1]						$ 502	$ 472	$ 445
Constellation Energy Group LLC [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment charges		$ 15	$ 119
Electricity Generation Plant, Non-Nuclear [Member]
Capital Leases Net Investment In Direct Financing Leases [Line Items]
Impairment of Long-Lived Assets to be Disposed of				$ 460

[1] (a)Includes interest expense to affiliates.