Commitments and Contingencies - Environmental Issues Narrative (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended		9 Months Ended				1 Months Ended		9 Months Ended		0 Months Ended		9 Months Ended			3 Months Ended
	Sep. 30, 2014 T	Dec. 31, 2013	Apr. 11, 2014 Commonwealth Edison Co [Member] Defendant	Jun. 30, 2013 Commonwealth Edison Co [Member]	Sep. 30, 2014 Commonwealth Edison Co [Member] MGPSite	Sep. 30, 2014 Midwest Generation, LLC [Member]	Mar. 31, 2014 Midwest Generation, LLC [Member]	Sep. 30, 2014 PECO Energy Co [Member] MGPSite	Sep. 30, 2014 Baltimore Gas and Electric Company [Member] MGPSite	Jan. 31, 2013 Exelon Generation Co L L C [Member]	Oct. 31, 2007 Exelon Generation Co L L C [Member]	Sep. 30, 2014 Exelon Generation Co L L C [Member] state	Dec. 31, 2013 Exelon Generation Co L L C [Member]	Apr. 12, 2012 Cotter Corporation [Member] Defendant	Feb. 28, 2012 Cotter Corporation [Member] Defendant	Sep. 30, 2014 Cotter Corporation [Member] Exelon Generation Co L L C [Member]	Sep. 30, 2014 Sixty-Eighth Street Dump [Member] Baltimore Gas and Electric Company [Member] Principle_responsible_party	Sep. 30, 2014 Rossville ash site [Member] Exelon Generation Co L L C [Member]	Sep. 30, 2014 Accrual For MGP Investigation And Remediation [Member] Commonwealth Edison Co [Member]	Sep. 30, 2014 Accrual For MGP Investigation And Remediation [Member] PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites					42			26	13
Approved clean-up					17			16
Sites under study/remediation					25			10	2
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)																			$ 26	$ 4
Low end of range of cooling tower cost												430
Consent decree penalty											1
Environmental loss contingencies							9					14	14
States subject to the Cross State Air Pollution Rule												28
Emissions allowance balance												66
Net investment in long-term direct financing leases	357	698
Payments for operating leases										10
Coal Rail Car Lease Proof of Claims				21
Increase (decrease) in the value of the asbestos liability reserve		25				6						15
Midwest Generation's estimated environmental investigation and remediation costs						53
Total cost of remediation to be shared by PRPs																42
DOJ potential settlement																90
Loss Contingency, Number of Defendants														14	15
Number Of Stations Violating Clean Air Act			6
Loss Contingency Number Of Parties Jointly And Severally Liable In Environmental Protection Agency Action																	19
Minimum estimated clean-up costs for all potentially responsible parties																	50
Maximum estimated clean-up costs for all potentially responsible parties									1.7								64.0
Loss Contingency, Estimate of Possible Loss																		$ 6
Minimum GHG emissions by stationary sources to qualify for regulation	100,000
Minimum additional GHG emissions by stationary sources after a modification	75,000
Effective Number Of Years For Tailoring Rule	6 years