Income Taxes - Recognition of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 462	$ 463	$ 523
Exelon Generation Co L L C [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	429	408	461
Commonwealth Edison Co [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
PECO Energy Co [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
Baltimore Gas and Electric Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1	0	0
Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	32	31	32
Potomac Electric Power Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
Delmarva Power & Light Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
Atlantic City Electric Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	$ 0	$ 0
State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	411
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	14
Tax Year 2019 [Member] | State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible	$ 21