Commitments and Contingencies - Environmental Issues Narrative (Details) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Aug. 31, 2015 gal	Oct. 31, 2007 USD ($)	Sep. 30, 2016 USD ($) MGPSite gal	Sep. 30, 2016 USD ($) Defendant MGPSite Open_claim gal
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Claims Dismissed, Number | Defendant				30
Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | MGPSite			17	17
Approved clean-up | MGPSite			42	42
Other Increase (Decrease) in Environmental Liabilities			$ 7
PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | MGPSite			16	16
Approved clean-up | MGPSite			26	26
Other Increase (Decrease) in Environmental Liabilities			$ 2
Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Approved clean-up | MGPSite			13	13
Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, New Claims Filed, Number | Open_claim				63
Site Contingency, Loss Exposure Not Accrued, Best Estimate			$ 90
Consent decree penalty		$ 1
Environmental loss contingencies			$ 13	$ 13
PEPCO Holdings Inc [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Sites under study/remediation | MGPSite			9	9
Environmental Remediation Expense				$ 10
PEPCO Holdings Inc [Member] | Minimum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			$ 7	7
PEPCO Holdings Inc [Member] | Maximum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			$ 18	18
Potomac Electric Power Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
QuantityOfMineralOilSPill | gal	6,100
Potomac Electric Power Company [Member] | Minimum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost				3
Potomac Electric Power Company [Member] | Maximum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost				$ 6
Delmarva Power and Light Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | gal			2	2
Calpine Corporation [Member] | Maximum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			$ 10	$ 10
Sixty-Eighth Street Dump [Member] | Baltimore Gas and Electric Company [Member] | Minimum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			50	50
Sixty-Eighth Street Dump [Member] | Baltimore Gas and Electric Company [Member] | Maximum [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			64	64
Rossville Ash Site [Member] | Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			6	6
Sauer Dump Site [Member] | Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss			$ 2	$ 2