Commitments and Contingencies - Narrative (Details) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) MGPSite gal	Dec. 31, 2016 MW	Jun. 30, 2018 USD ($) MGPSite Open_claim gal MW shares	Jun. 30, 2017 shares	Dec. 31, 2017 USD ($) Open_claim	Sep. 30, 2017 USD ($) MGPSite
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
BusinessAcquisitionResidualcommitment | MW			55
Guarantor Obligations, Maximum Exposure, Undiscounted	58		$ 58
Loss Contingency, Loss in Period			$ 50
Power Volume | MW		10	220
Open Asbestos Related Personal Injury Claims | Open_claim					224
Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Estimated Insurance Recoveries	3,200		3,200
Accrued Insurance, Current	13,100		13,100
Prepaid Reinsurance Premiums	350		350
Loss Contingency, Undiscounted Amount of Insurance-related Assessment Liability	450
Maximum liability per nuclear incident	12,600		12,600
Costs Incurred, Development Costs	458
Accrual for Environmental Loss Contingencies, Component Amount						$ 90
Liability for Asbestos and Environmental Claims, Gross	80		80		78
Liability for Asbestos and Environmental Claims, Net	22		22
Liability for Asbestos and Environmental Claims, Net, Payment for Claims	58
Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Environmental Remediation Expense			10
Number of Real Estate Properties | MGPSite						9
Commonwealth Edison Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Mgp Site Contingency Approved Cleanup Count | MGPSite	42		42
Mgp Site Contingency Count | MGPSite	20		20
Mgp Site Contingency Monitoring Count | MGPSite	22		22
Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Guarantor Obligations, Maximum Exposure, Undiscounted	17		17
Accrued Liabilities and Other Liabilities	30		30		28
PECO Energy Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Mgp Site Contingency Approved Cleanup Count | MGPSite	26		26
Mgp Site Contingency Count | MGPSite	17		17
Mgp Site Contingency Monitoring Count | MGPSite	9		9
Baltimore Gas and Electric Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Mgp Site Contingency Approved Cleanup Count | MGPSite	13		13
Mgp Site Contingency Monitoring Count | MGPSite	4		4
Delmarva Power and Light Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 24		$ 24
Mgp Site Contingency Approved Cleanup Count | gal	3		3
Mgp Site Contingency Count | MGPSite	2		2
Maximum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, New Claims Filed, Number | Open_claim					20,000,000
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] | Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss						$ 7
Nuclear Insurance Premiums [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Nuclear financial protection pool value	$ 2,800		$ 2,800
Nuclear Insurance Premiums [Member] | Maximum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Nuclear financial protection pool value	$ 420		$ 420
Pepco Holdings [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | shares			0	0
West Lake [Member] | Minimum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, New Claims Filed, Number | Open_claim			340,000,000