Property, Plant and Equipment Property, Plant and Equipment - Summary of Capitalized Interest and AFUDC (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Mar. 23, 2016	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			$ 1,695	$ 1,658	$ 1,678
Interest Costs Capitalized				31	63	108
Allowance for Funds Used During Construction, Capitalized Interest				109	108	98
Pepco Holdings LLC [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			305	263
Allowance for Funds Used During Construction, Capitalized Interest				44	54
Exelon Generation Co L L C [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			464	502	472
Interest Costs Capitalized				31	63	107
Allowance for Funds Used During Construction, Capitalized Interest				0	0	0
Commonwealth Edison Co [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			377	369	469
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				30	20	22
PECO Energy Co [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			141	130	127
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				12	12	11
Baltimore Gas and Electric Company [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			130	111	114
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				24	22	30
Potomac Electric Power Company [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			162	133	137
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				34	34	29
Delmarva Power & Light Company [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			62	54	52
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				4	10	7
Atlantic City Electric Company [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]			68	64	65
Interest Costs Capitalized				0	0	0
Allowance for Funds Used During Construction, Capitalized Interest				$ 4	$ 9	$ 9
Successor [Member] | Pepco Holdings LLC [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]		$ 207
Allowance for Funds Used During Construction, Capitalized Interest			$ 35
Predecessor [Member] | Pepco Holdings LLC [Member]
Capitalized Interest And AFUDC [Line Items]
Interest Costs Incurred	[1]	$ 68
Allowance for Funds Used During Construction, Capitalized Interest		$ 10

[1] Includes interest expense to affiliates.