Commitments and Contingencies - Schedule of Accruals for Environmental Matters (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	$ 462	$ 466
Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	117	117
Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	283	285
PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	29	30
Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	5	5
PEPCO Holdings Inc [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	28	29
Potomac Electric Power Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Accrual for Environmental Loss Contingencies, Gross	26	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	313	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	281	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	28	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 Inc [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