Income Taxes - Narrative (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2009
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%	21.00%	35.00%
BonusDepreciationPercentage							50.00%
AllowableExpensingForCostOfQualifiedProperty						30.00%
LimitationonDeductibleInterestExpense				100.00%
Regulatory Liabilities			$ 10,246	$ 10,388			$ 10,388
Unrecognized Tax Benefits			733	743			743
Like kind exchange [Member]
Income Tax Additional Narrative Information [Line Items]
Deferred Gain on Sale of Property								$ 1,200
Decrease in Unrecognized Tax Benefits is Reasonably Possible			33
IRS asserted penalties for understatement of tax			$ 90
Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Unrecognized Tax Benefits			$ 464	468			468
Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 6,424	6,577			6,577
Unrecognized Tax Benefits			2	2			2
Commonwealth Edison Co [Member] | Like kind exchange [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible			$ 2
PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 646	690			690
Unrecognized Tax Benefits			$ 0	0			0
Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 1,185	1,163			1,163
Unrecognized Tax Benefits			$ 120	120			120
Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 1,965	1,928			1,928
Unrecognized Tax Benefits			$ 125	125			125
Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 845	832			832
Unrecognized Tax Benefits			$ 59	59			59
Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 646	635			635
Unrecognized Tax Benefits			$ 21	21			21
Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liabilities			$ 437	422			422
Unrecognized Tax Benefits			14	14			14
State and Local Jurisdiction [Member]
Income Tax Additional Narrative Information [Line Items]
Unrecognized tax benefits that if recognized would affect the effective tax rate			0
Decrease in Unrecognized Tax Benefits is Reasonably Possible			679
Increase (Decrease) in Other Deferred Liability		$ 17
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Unrecognized tax benefits that if recognized would affect the effective tax rate			458
Decrease in Unrecognized Tax Benefits is Reasonably Possible			465
State and Local Jurisdiction [Member] | Exelon Generation Co L L C [Member] | Subsequent Event [Member]
Income Tax Additional Narrative Information [Line Items]
Increase (Decrease) in Other Deferred Liability		5
State and Local Jurisdiction [Member] | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Deferred Tax Assets, State Taxes			272
State and Local Jurisdiction [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible			120
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member] | Subsequent Event [Member]
Income Tax Additional Narrative Information [Line Items]
Increase (Decrease) in Other Deferred Liability		18
State and Local Jurisdiction [Member] | Pepco Holdings LLC [Member] | Successor [Member]
Income Tax Additional Narrative Information [Line Items]
Deferred Tax Assets, State Taxes					$ 8
Decrease in Unrecognized Tax Benefits is Reasonably Possible			94
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Deferred Tax Assets, State Taxes					$ 8
Decrease in Unrecognized Tax Benefits is Reasonably Possible			59
State and Local Jurisdiction [Member] | Potomac Electric Power Company [Member] | Subsequent Event [Member]
Income Tax Additional Narrative Information [Line Items]
Increase (Decrease) in Other Deferred Liability		17
State and Local Jurisdiction [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible			21
Increase (Decrease) in Other Deferred Liability		$ 1
State and Local Jurisdiction [Member] | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Decrease in Unrecognized Tax Benefits is Reasonably Possible			14
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		7,315
Increase (Decrease) in Other Deferred Liability	[2]		8,624
Deferred Income Tax Expense (Benefit)	[2]		1,309
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Exelon Generation Co L L C [Member]
Income Tax Additional Narrative Information [Line Items]
Increase (Decrease) in Other Deferred Liability			1,895
Deferred Income Tax Expense (Benefit)			1,895
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		2,818
Increase (Decrease) in Other Deferred Liability			2,819
Deferred Income Tax Expense (Benefit)			1
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1],[3]		1,394
Increase (Decrease) in Other Deferred Liability			1,407
Deferred Income Tax Expense (Benefit)			13
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		1,124
Increase (Decrease) in Other Deferred Liability			1,120
Deferred Income Tax Expense (Benefit)			(4)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		1,979
Increase (Decrease) in Other Deferred Liability			1,944
Deferred Income Tax Expense (Benefit)			(35)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		976
Increase (Decrease) in Other Deferred Liability			968
Deferred Income Tax Expense (Benefit)			(8)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		545
Increase (Decrease) in Other Deferred Liability			540
Deferred Income Tax Expense (Benefit)			(5)
RemeasurementOfDeferredIncomeTaxesDueToTaxReform [Member] | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[1]		458
Increase (Decrease) in Other Deferred Liability			456
Deferred Income Tax Expense (Benefit)			$ (2)
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				3,040			3,040
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				1,400			1,400
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[4]			533			533
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				459			459
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				648			648
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				299			299
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				195			195
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToIRSNormalizationRules [Member] | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				153			153
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				1,694			1,694
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[4]			573			573
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[4]			43			43
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				324			324
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				754			754
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				391			391
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				194			194
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomersSubjectToRateRegulatorDeterminations [Member] | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				170			170
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				4,734			4,734
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Commonwealth Edison Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				1,973			1,973
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | PECO Energy Co [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities	[4]			576			576
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Baltimore Gas and Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				783			783
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Pepco Holdings LLC [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				1,402			1,402
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Potomac Electric Power Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				690			690
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Delmarva Power and Light Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				389			389
NetRegulatoryLiabilitiesAssociatedWithUltimateSettlementToCustomers [Member] | Atlantic City Electric Company [Member]
Income Tax Additional Narrative Information [Line Items]
Regulatory Liabilities				$ 323			$ 323

[1] Reflects the net regulatory liabilities recorded on a pre-tax basis before taking into consideration the income tax benefits associated with the ultimate settlement with customers.

[2] Amounts do not sum across due to deferred tax adjustments recorded at the Exelon Corporation parent company, primarily related to certain employee compensation plans.

[3] Given the regulatory treatment of income tax benefits related to electric and gas distribution repairs, PECO remains in an overall net regulatory asset position as of December 31, 2017 after recording the impacts related to the TCJA. Refer to Note 3 - Regulatory Matters for additional information.

[4] Given the regulatory treatment of income tax benefits related to electric and gas distribution repairs, PECO remains in an overall net regulatory asset position as of December 31, 2017 after recording the impacts related to the TCJA. As a result, the amount of customer benefits resulting from the TCJA subject to the discretion of PECO's rate regulators are lower relative to the other Utility Registrants. Refer to Note 3 - Regulatory Matters for additional information.