Supplemental Financial Information Supplemental Financial Information - Utility Taxes (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Mar. 23, 2016	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	$ 245	$ 255			$ 682	$ 624
Exelon Generation Co L L C [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	35	35			97	90
Commonwealth Edison Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	65	67			181	186
PECO Energy Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	35	40			95	106
Baltimore Gas and Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	22	21			69	66
Pepco Holdings LLC [Member] | Successor [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	88	92		$ 176	240
Pepco Holdings LLC [Member] | Predecessor [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes			$ 78
Potomac Electric Power Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	83	87			226	240
Delmarva Power and Light Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	5	5			14	14
Atlantic City Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	$ 0	$ 0			$ 0	$ 0