Commitments and Contingencies - Narrative (Details)	1 Months Ended			3 Months Ended		9 Months Ended	12 Months Ended
	Nov. 30, 2015 USD ($)	Sep. 23, 2015 USD ($)	Aug. 31, 2015 gal	Sep. 30, 2017 USD ($) MGPSite gal	Dec. 31, 2016 USD ($) Open_claim	Sep. 30, 2017 USD ($) MGPSite Open_claim gal MW	Dec. 31, 2016 USD ($) Open_claim	Nov. 29, 2016 employee	Jun. 30, 2014 USD ($)
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies									$ 1,000,000,000
BusinessAcquisitionResidualcommitment | MW						55
Guarantor Obligations, Maximum Exposure, Undiscounted				$ 49,000,000		$ 49,000,000
Loss Contingency, Loss in Period						$ 50,000,000
Power Volume | MW						220
Open Asbestos Related Personal Injury Claims | Open_claim					227		227
PEPCO Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Environmental Remediation Expense						$ 10,000,000
Number of Real Estate Properties | MGPSite				9		9
Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Consent Decree Penalty						$ 1,600,000
Payments for Legal Settlements						1,600,000
Commitments and Contingencies
Estimated Insurance Recoveries				3,200,000,000		3,200,000,000
Accrued Insurance, Current				13,400,000,000		13,400,000,000
Prepaid Reinsurance Premiums				360,000,000		360,000,000
Loss Contingency, Undiscounted Amount of Insurance-related Assessment Liability				450,000,000
Maximum liability per nuclear incident				13,000,000,000		13,000,000,000
Entity Number of Employees | employee								1,500
Leasehold Improvements, Gross					110,000,000		110,000,000
Costs Incurred, Development Costs				457,000,000
Accrual for Environmental Loss Contingencies, Component Amount				90,000,000		90,000,000
Liability for Asbestos and Environmental Claims, Gross				80,000,000	83,000,000	80,000,000	83,000,000
Liability for Asbestos and Environmental Claims, Net				22,000,000		22,000,000
Liability for Asbestos and Environmental Claims, Net, Payment for Claims				58,000,000
Loss Contingency, Damages Sought, Value							14,000,000
Commonwealth Edison Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Increase (Decrease) in Environmental Liabilities				13,000,000
Commitments and Contingencies
Mgp Site Contingency Approved Cleanup Count | MGPSite				42		42
Mgp Site Contingency Count | MGPSite				19		19
Mgp Site Contingency Monitoring Count | MGPSite				23		23
Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted				$ 14,000,000		$ 14,000,000
QuantityOfMineralOilSPill | gal			6,100
PECO Energy Co [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Other Increase (Decrease) in Environmental Liabilities				2,000,000
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]
Occupancy, Net	$ 42,000,000	$ 12,000,000
Operating Leases, Income Statement, Initial Direct Costs					25,000,000
Oil and Gas Property, Lease Operating Expense					29,000,000
Operating Expenses					$ 28,000,000
Mgp Site Contingency Approved Cleanup Count | MGPSite				13		13
Mgp Site Contingency Monitoring Count | MGPSite				2		2
Delmarva Power and Light Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted				$ 19,000,000		$ 19,000,000
Mgp Site Contingency Approved Cleanup Count | gal				3		3
Mgp Site Contingency Count | MGPSite				2		2
Maximum [Member] | PEPCO Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss				$ 18,000,000		$ 18,000,000
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						650,000,000
Maximum [Member] | Calpine Corporation [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss				10,000,000		$ 10,000,000
Maximum [Member] | Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Environmental Exit Costs, Anticipated Cost						6,000,000
Minimum [Member] | PEPCO Holdings Inc [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss				7,000,000		$ 7,000,000
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						225,000,000
Minimum [Member] | Potomac Electric Power Company [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Environmental Exit Costs, Anticipated Cost						$ 3,000,000
Nuclear Plant [Member] | Maximum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies				500,000,000		500,000,000
Nuclear Plant [Member] | Minimum [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Commitments and Contingencies				450,000,000		450,000,000
Nuclear Insurance Premiums [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Nuclear financial protection pool value				2,800,000,000		2,800,000,000
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				400,000,000		400,000,000
Rossville Ash Site [Member] | Exelon Generation Co L L C [Member]
Commitments And Contingencies Additional Narrative Information [Line Items]
Loss Contingency, Estimate of Possible Loss				$ 1,000,000		$ 1,000,000