Commitments and Contingencies - Narrative (Details) shares in Millions, number in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($) Open_claim MW shares	Mar. 31, 2017 shares	Dec. 31, 2016 MW	Dec. 31, 2017 USD ($) Open_claim	Mar. 31, 2018	Mar. 31, 2018 MGPSite	Mar. 31, 2018 gal	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
Loss Contingency, Loss in Period	$ 50
Power Volume | MW	220		10
Open Asbestos Related Personal Injury Claims | Open_claim				232
Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies
Estimated Insurance Recoveries	3,200
Accrued Insurance, Current	13,200
Prepaid Reinsurance Premiums	360
Loss Contingency, Undiscounted Amount of Insurance-related Assessment Liability	450
Maximum liability per nuclear incident	13,000
Costs Incurred, Development Costs	458
Accrual for Environmental Loss Contingencies, Component Amount								$ 90
Liability for Asbestos and Environmental Claims, Gross	76			78
Liability for Asbestos and Environmental Claims, Net	21
Liability for Asbestos and Environmental Claims, Net, Payment for Claims	55
Pepco Holdings LLC [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Environmental Remediation Expense	10
Number of Real Estate Properties | MGPSite								9
Commonwealth Edison Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Mgp Site Contingency Approved Cleanup Count | MGPSite						42
Mgp Site Contingency Count | MGPSite						20
Mgp Site Contingency Monitoring Count | MGPSite						22
Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	17
Accrued Liabilities and Other Liabilities	30			$ 28
PECO Energy Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Mgp Site Contingency Approved Cleanup Count | MGPSite						26
Mgp Site Contingency Count | MGPSite						17
Mgp Site Contingency Monitoring Count | MGPSite						9
Baltimore Gas and Electric Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Mgp Site Contingency Approved Cleanup Count | MGPSite						13
Mgp Site Contingency Monitoring Count					13	4
Delmarva Power and Light Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	24
Mgp Site Contingency Approved Cleanup Count | gal							3
Mgp Site Contingency Count | MGPSite						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
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
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