Supplemental Financial Information Supplemental Financial Information - Utility Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	$ 253	$ 245	$ 705	$ 682
Exelon Generation Co L L C [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	32	35	92	97
Commonwealth Edison Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	67	65	188	181
PECO Energy Co [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	39	35	102	95
Baltimore Gas and Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	23	22	70	69
Pepco Holdings LLC [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	92	88	253	240
Potomac Electric Power Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	87	83	238	226
Delmarva Power and Light Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	5	5	15	14
Atlantic City Electric Company [Member]
Supplemental Income Statement Information [Line Items]
Utilities Operating Expense, Taxes	$ 0	$ 0	$ 0	$ 0