Commitments and Contingencies - Narrative (Details) (USD $)	0 Months Ended	3 Months Ended	12 Months Ended			0 Months Ended		1 Months Ended		3 Months Ended	12 Months Ended	0 Months Ended			3 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended
	Jun. 05, 2013 Customer	Jun. 30, 2013	Dec. 31, 2014 Open_claim		Dec. 31, 2012	Apr. 12, 2012 Defendant	Feb. 28, 2012 Defendant	Jan. 31, 2013	Oct. 31, 2007	Jun. 30, 2014	Dec. 31, 2013	Jan. 31, 2005	Nov. 19, 2013 Defendant	Jul. 11, 2011 Customer	Sep. 30, 2014	May 31, 2006 Principle_responsible_party	Dec. 31, 1999 Principle_responsible_party	Jan. 22, 2015	Jul. 26, 2013 MW	Dec. 27, 2013 MW	Dec. 12, 2014 MW	Dec. 22, 2014 MW	Mar. 31, 2014	Dec. 31, 2003	May 30, 2012
Commitments and Contingencies [Line Items]
Nuclear insurance liability limit per incident			$ 18,300,000.0
Cost surcharge to Price-Anderson Act nuclear incident assessment			5.00%
Guarantee obligations maximum exposure			9,402,000,000
Coal rail car lease proof of claims		21,000,000
Maximum estimated clean-up costs for all potentially responsible parties			105,000,000
Asbestos liability reserve			100,000,000								90,000,000
Probable contingency (liability)			22,000,000
Open asbestos liability claims			255
Asbestos liability reserve related to anticipated claims			78,000,000
FERC civil penalty			110,000,000
FERC settlement recorded in operation and maintenance expense					50,000,000
Number of customers affected by a major storm	34,559
Rossville ash site [Member]
Commitments and Contingencies [Line Items]
Area of property			32
Cotter Corporation [Member]
Commitments and Contingencies [Line Items]
Number of defendants						14	15
Nuclear Insurance Premiums [Member]
Commitments and Contingencies [Line Items]
Guarantee obligations maximum exposure			3,014,000,000	[1]
Sithe Guarantee [Member]
Commitments and Contingencies [Line Items]
Guarantee obligations maximum exposure											200,000,000
Exelon Generation Co L L C [Member]
Commitments and Contingencies [Line Items]
Nuclear financial protection pool value			375,000,000
United States licensed nuclear reactors total			104
Maximum recovery limit from a nuclear industry mutual insurance company in the event of multiple losses			13,200,000,000.0
Cost surcharge to Price-Anderson Act nuclear incident assessment			12730000000.00%
Maximum assessment mandated by Price-Anderson Act per nuclear reactor for a nuclear incident			19,000,000
Maximum annual assessment payment mandated by Price-Anderson Act for a nuclear incident			2,700,000,000
Total retrospective premium obligation under insurance from a nuclear industry mutual insurance company			319,000,000
Mutual Replacement Power Cost Insurance Maximum Retrospective Premium Obligation			3,200,000,000
Cost of spent nuclear fuel disposal per kWh of net nuclear generation			0.001
Net nuclear fuel disposal fees			49,000,000								136,000,000
Department of Energy SNF one-time fee applicable to nuclear generation			277,000,000
DOE SNF one-time fee with interest			1,021,000,000
13-week Treasury Rate used to calculate DOE SNF one-time fee			2.00%
Capacity of energy construction project			150
Guarantee obligations maximum exposure			6,384,000,000
Consent decree penalty									1,000,000
Environmental loss contingencies			13,000,000								14,000,000
Payments for operating leases								10,000,000
Increase (decrease) in the value of the asbestos liability reserve										15,000,000	25,000,000
Open asbestos liability claims			300
FERC civil penalty			135,000,000
FERC settlement recorded in operation and maintenance expense					195,000,000
Exelon Generation Co L L C [Member] | Cotter Corporation [Member]
Commitments and Contingencies [Line Items]
Total cost of remediation to be shared by PRPs			50,000,000
DOJ potential settlement			90,000,000
Exelon Generation Co L L C [Member] | Rossville ash site [Member]
Commitments and Contingencies [Line Items]
Estimate of possible loss			10,000,000
Exelon Generation Co L L C [Member] | Equity Method Investments [Member]
Commitments and Contingencies [Line Items]
Purchase obligations, due within two years			38,000,000
Unrecorded unconditional purchase obligation, due in next 12 months			98,000,000
Other unrecorded amounts			20,000,000
Exelon Generation Co L L C [Member] | Perryman Construction [Member]
Commitments and Contingencies [Line Items]
Purchase obligations, due within two years																			39,000,000
Capacity of energy construction project																			120
Exelon Generation Co L L C [Member] | Fourmile Wind Project [Member]
Commitments and Contingencies [Line Items]
Capacity of energy construction project			40																	40
Unrecorded unconditional purchase obligation, due in next 12 months																				2,000,000
Exelon Generation Co L L C [Member] | Combine-cycle Turbine Units [Member]
Commitments and Contingencies [Line Items]
Unrecorded unconditional purchase obligation, due in next 12 months			1,000,000,000
Exelon Generation Co L L C [Member] | Fair Wind Project [Member]
Commitments and Contingencies [Line Items]
Capacity of energy construction project			30																		30
Unrecorded unconditional purchase obligation, due in next 12 months																					19,000,000
Exelon Generation Co L L C [Member] | Sendero Wind Project [Member]
Commitments and Contingencies [Line Items]
Capacity of energy construction project																						78
Unrecorded unconditional purchase obligation, due in next 12 months																						56,000,000
Exelon Generation Co L L C [Member] | Beebe Construction [Member]
Commitments and Contingencies [Line Items]
Accrued environmental liabilities			2,000,000
Exelon Generation Co L L C [Member] | Nuclear Insurance Premiums [Member]
Commitments and Contingencies [Line Items]
Nuclear insurance liability limit per incident			13,600,000,000.0
Guarantee obligations maximum exposure			3,014,000,000	[2]
Exelon Generation Co L L C [Member] | Guarantees Other Than Letters Of Credit and Nuclear Insurance Premiums [Member]
Commitments and Contingencies [Line Items]
Mutual Property Insurance Distribution To Members			18,500,000
Exelon Generation Co L L C [Member] | Sithe Guarantee [Member]
Commitments and Contingencies [Line Items]
Acquisition of interest in subsidiary												50.00%
Sale of interest in subsidiary												100.00%
Midwest Generation, LLC [Member]
Commitments and Contingencies [Line Items]
Environmental loss contingencies																							9,000,000
Percentage agreed to reimburse related parties																								50.00%
Increase (decrease) in the value of the asbestos liability reserve										6,000,000
Midwest Generation's estimated environmental investigation and remediation costs																							53,000,000
Commonwealth Edison Co [Member]
Commitments and Contingencies [Line Items]
Guarantee obligations maximum exposure			222,000,000
Total number of MGP sites			42
Sites approved for clean-up			17
Sites under study/remediation			25
Minimum number of customers ComEd can be held liable to for power interruption			30,000
Number of customers affected by a major storm														900,000
Number of possible defendants													1,200,000
Interest rate on long-term debt			6.35%	[3]
Commonwealth Edison Co [Member] | Minimum [Member]
Commitments and Contingencies [Line Items]
Value of damages sought													500
Commonwealth Edison Co [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Value of damages sought													1,500
Commonwealth Edison Co [Member] | Accrual For MGP Investigation And Remediation [Member]
Commitments and Contingencies [Line Items]
Increase (decrease) in accrual for environmental loss contingencies															26,000,000
PECO Energy Co [Member]
Commitments and Contingencies [Line Items]
Guarantee obligations maximum exposure			218,000,000
Total number of MGP sites			26
Sites approved for clean-up			16
Sites under study/remediation			10
PECO Energy Co [Member] | Accrual For MGP Investigation And Remediation [Member]
Commitments and Contingencies [Line Items]
Increase (decrease) in accrual for environmental loss contingencies															4,000,000
Baltimore Gas and Electric Company [Member]
Commitments and Contingencies [Line Items]
Guarantee obligations maximum exposure			265,000,000
Total number of MGP sites			13
Sites under study/remediation			2
Maximum estimated clean-up costs for all potentially responsible parties																									1,700,000.0
Number of claimants			486
Baltimore Gas and Electric Company [Member] | Deferrable Interest Subordinated Debentures [Member]
Commitments and Contingencies [Line Items]
Interest rate on long-term debt			6.20%
Baltimore Gas and Electric Company [Member] | Sixty-Eighth Street Dump [Member]
Commitments and Contingencies [Line Items]
Number of parties jointly and severally liable In environmental protection agency action																19	19
Minimum estimated clean-up costs for all potentially responsible parties			50,000,000
Maximum estimated clean-up costs for all potentially responsible parties			64,000,000
Subsequent Event [Member]
Commitments and Contingencies [Line Items]
Settlement amount																		$ 14,000,000

[1] Nuclear insurance premiums—Represents the maximum amount that Generation would be required to pay for retrospective premiums in the event of nuclear disaster at any domestic site under the Secondary Financial Protection pool as required under the Price-Anderson Act as well as the current aggregate annual retrospective premium obligation that could be imposed by NEIL. See the Nuclear Insurance section within this note for additional details on Generation’s nuclear insurance premiums.

[2] Nuclear insurance premiums — Represents the maximum amount that Generation would be required to pay for retrospective premiums in the event of nuclear disaster at any domestic site, including CENG sites, under the Secondary Financial Protection pool as required under the Price-Anderson Act as well as the current aggregate annual retrospective premium obligation that could be imposed by NEIL. See the Nuclear Insurance section within this note for additional details on Generation’s nuclear insurance premiums.

[3] Amount owed to this financing trust is recorded as Long-term debt to financing trust within ComEd’s Consolidated Balance Sheets.