Commitments and Contingencies - Schedule of Accruals for Environmental Matters (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	$ 486	$ 466
Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	102	117
Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	323	285
PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	28	30
Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	6	5
Pepco Holdings LLC [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	27	29
Potomac Electric Power Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	25	27
Delmarva Power and Light Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	1	1
Atlantic City Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	1	1
Accrual For MGP Investigation And Remediation [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	352	315
Accrual For MGP Investigation And Remediation [Member] | Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Accrual For MGP Investigation And Remediation [Member] | Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	321	283
Accrual For MGP Investigation And Remediation [Member] | PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	27	28
Accrual For MGP Investigation And Remediation [Member] | Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	4	4
Accrual For MGP Investigation And Remediation [Member] | Pepco Holdings LLC [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Accrual For MGP Investigation And Remediation [Member] | Potomac Electric Power Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Accrual For MGP Investigation And Remediation [Member] | Delmarva Power and Light Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	0	0
Accrual For MGP Investigation And Remediation [Member] | Atlantic City Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	$ 0	$ 0