Regulatory Matters - Annual Electric Transmission Filings (Details) $ in Millions					3 Months Ended		12 Months Ended
	Dec. 05, 2019 USD ($)	Nov. 26, 2019 USD ($)	May 23, 2019 USD ($)	Jun. 27, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 4	$ 24
Commonwealth Edison Co [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 50					$ 53	$ (3)
Public Utilities, Requested Rate Increase (Decrease), Amount							21
Annual Reconciliation Increase (Decrease)							(16)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount			$ 51				$ 5
Public Utilities, Approved Equity Capital Structure, Percentage							8.21%
Public Utilities, Approved Return on Equity, Percentage		8.91%					11.50%
Additional basis points for RTO membership incentive							50
Cap on Equity Component of Rate of Return on Common Equity in Federal Energy Regulatory Committee Complaint							55.00%
PECO Energy Co [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 14			$ 22
Annual Reconciliation Increase (Decrease)							$ 8	$ (6)
Public Utilities, Approved Equity Capital Structure, Percentage	7.62%
Additional basis points for RTO membership incentive	50			50
Baltimore Gas and Electric Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							(10)
Annual Reconciliation Increase (Decrease)							(23)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount							$ (19)
Public Utilities, Approved Equity Capital Structure, Percentage							7.35%
Public Utilities, Approved Return on Equity, Percentage							10.50%
Additional basis points for RTO membership incentive							50
Potomac Electric Power Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 15
Annual Reconciliation Increase (Decrease)							11
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount							$ 26
Public Utilities, Approved Equity Capital Structure, Percentage							7.75%
Public Utilities, Approved Return on Equity, Percentage							10.50%
Additional basis points for RTO membership incentive							50
Delmarva Power & Light Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 17
Annual Reconciliation Increase (Decrease)							(1)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount							$ 16
Public Utilities, Approved Equity Capital Structure, Percentage							7.14%
Public Utilities, Approved Return on Equity, Percentage							10.50%
Additional basis points for RTO membership incentive							50
Atlantic City Electric Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 11
Annual Reconciliation Increase (Decrease)							(2)
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount							$ 9
Public Utilities, Approved Equity Capital Structure, Percentage							7.79%
Public Utilities, Approved Return on Equity, Percentage							10.50%
Additional basis points for RTO membership incentive							50
Electric Transmission [Member] | Baltimore Gas and Electric Company [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount							$ 14