Commitments and Contingencies Commitments and Contingencies - Schedule of Environmental Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	$ 466	$ 429
Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	315	325
Exelon Generation Co L L C [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	117	72
Exelon Generation Co L L C [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Commonwealth Edison Co [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	285	292
Commonwealth Edison Co [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	283	291
PECO Energy Co [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	30	33
PECO Energy Co [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	28	31
Baltimore Gas and Electric Company [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	5	2
Baltimore Gas and Electric Company [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	4	2
Pepco Holdings LLC [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	29	30
Pepco Holdings LLC [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	1
Potomac Electric Power Company [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	27	27
Potomac Electric Power Company [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Delmarva Power and Light Company [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	1	2
Delmarva Power and Light Company [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	1
Atlantic City Electric Company [Member] | Total Accrual For Environmental Loss Contingencies [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	1	1
Atlantic City Electric Company [Member] | Accrual For MGP Investigation And Remediation [Member]
Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	$ 0	$ 0