Income Taxes - Settlements of Income Tax Audits, Refund Claims, and Litigation (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
State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	425
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	411
Exelon Generation Co LLC Affiliate [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	411
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
Exelon Generation Co L L C [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	411
Commonwealth Edison Co [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
Commonwealth Edison Co [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
PECO Energy 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] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	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
Baltimore Gas and Electric Company [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
Pepco Holdings LLC [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	32	31	32
Pepco Holdings LLC [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	0
Potomac Electric Power Company [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	0	0	0
Potomac Electric Power Company [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	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
Delmarva Power & Light Company [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	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
Atlantic City Electric Company [Member] | State and Local Jurisdiction [Member]
Recognition of Unrecognized Tax Benefits [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 14