Commitments and Contingencies - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) MGPSite Open_claim gal	Dec. 31, 2016 MW	Sep. 30, 2018 USD ($) MGPSite Open_claim gal MW	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 3,386		$ 3,386
Costs Incurred, Development Costs			$ 458
Power Volume | MW		10	220
Business Acquisition Residual Commitment | MW			55
Loss Contingency, Loss in Period			$ 50
Accrual for Environmental Loss Contingencies, Gross	486		486	$ 466
Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	2,766		2,766
Costs Incurred, Development Costs			458
Loss Contingency, Loss in Period			50
Accrued Insurance, Current	13,100		13,100
Loss Contingency, Undiscounted Amount of Insurance-related Assessment Liability			450
Maximum liability per nuclear incident	12,600		12,600
Prepaid Reinsurance Premiums	345		345
Estimated Insurance Recoveries	3,200		3,200
Site Contingency, Loss Exposure Not Accrued, Best Estimate			90
Accrual for Environmental Loss Contingencies, Gross	102		102	117
Accrual for Environmental Loss Contingencies, Component Amount	90		90
Liability for Asbestos and Environmental Claims, Gross	$ 80		$ 80	78
Open Asbestos Related Personal Injury Claims | Open_claim	241		241
Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 91		$ 91
Accrual for Environmental Loss Contingencies, Gross	$ 27		$ 27	29
Number of Real Estate Properties | MGPSite	9		9
Environmental Remediation Expense			$ 10
Commonwealth Edison Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 211		$ 211
Mgp Site Contingency Approved Cleanup Count | MGPSite	42		42
Mgp Site Contingency Count | MGPSite	20		20
Mgp Site Contingency Monitoring Count | MGPSite	22		22
Other Increase (Decrease) in Environmental Liabilities	$ 48
Accrual for Environmental Loss Contingencies, Gross	323		$ 323	285
Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	43		43
Accrual for Environmental Loss Contingencies, Gross	25		25	27
PECO Energy Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 187		$ 187
Mgp Site Contingency Approved Cleanup Count | MGPSite	26		26
Mgp Site Contingency Count | MGPSite	17		17
Mgp Site Contingency Monitoring Count | MGPSite	9		9
Accrual for Environmental Loss Contingencies, Gross	$ 28		$ 28	30
Baltimore Gas and Electric Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 28		$ 28
Mgp Site Contingency Approved Cleanup Count | MGPSite	13		13
Mgp Site Contingency Count | MGPSite	9		9
Mgp Site Contingency Monitoring Count | MGPSite	4		4
Accrual for Environmental Loss Contingencies, Gross	$ 6		$ 6	5
Delmarva Power and Light Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 13		$ 13
Mgp Site Contingency Approved Cleanup Count | gal	3		3
Mgp Site Contingency Count | MGPSite	2		2
Accrual for Environmental Loss Contingencies, Gross	$ 1		$ 1	$ 1
Open Claims [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Liability for Asbestos and Environmental Claims, Gross	24		24
Estimated Future Claims [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Liability for Asbestos and Environmental Claims, Gross	56		$ 56
Maximum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Business Acquisition, Expected New Generation Mwh | MW			300
Maximum [Member] | Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss					$ 18
Maximum [Member] | Calpine Corporation [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss					10
Minimum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Business Acquisition, Expected New Generation Mwh | MW			285
Minimum [Member] | Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss					$ 7
Nuclear Insurance Premiums [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Nuclear financial protection pool value	2,800		$ 2,800
Nuclear Insurance Premiums [Member] | Maximum [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Nuclear financial protection pool value	420		420
Guarantees Other Than Letters Of Credit and Nuclear Insurance Premiums [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Mutual Property Insurance Distribution To Members	31		31
Guarantees Other Than Letters Of Credit and Nuclear Insurance Premiums [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Mutual Property Insurance Distribution To Members	31		31
Pepco Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	513		513
Pepco Holdings Inc [Member] | Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	315		315
Pepco Holdings Inc [Member] | Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	120		120
Pepco Holdings Inc [Member] | Delmarva Power and Light Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	84		84
West Lake [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Accrual for Environmental Loss Contingencies, Gross	280		280
West Lake [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Accrual for Environmental Loss Contingencies, Gross	20		$ 20
Other Solar Projects [Member] | Pepco Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Business Acquisition, Expected New Generation Mwh | MW			37
Other Non-Financial Commitment | MW			27
Other Commitment	137		$ 137
Other Solar Projects [Member] | Pepco Holdings Inc [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Commitment	$ 137		$ 137
Wind Assets [Member] | Pepco Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Non-Financial Commitment | MW			100
Purchase Commitment | MW			120