Commitments and Contingencies - Environmental Issues Narrative (Details) $ in Millions			1 Months Ended		3 Months Ended	6 Months Ended
	Apr. 12, 2012 Defendant	Feb. 28, 2012 Defendant	Jan. 31, 2015 USD ($)	Oct. 31, 2007 USD ($)	Jun. 30, 2015 USD ($) MGPSite	Jun. 30, 2015 USD ($) Principle_responsible_party MGPSite	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Accrual For Environmental Loss Contingencies [Line Items]
Net investment in long-term direct financing leases					$ 344.0	$ 344.0	$ 361.0
Litigation Settlement, Amount			$ 14.0
Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | MGPSite					42	42
Approved clean-up | MGPSite					17	17
Sites under study/remediation | MGPSite					25	25
PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | MGPSite					26	26
Approved clean-up | MGPSite					16	16
Sites under study/remediation | MGPSite					10	10
Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites | MGPSite					13	13
Sites under study/remediation | MGPSite					2	2
Loss Contingency, Estimate of Possible Loss					$ 1.7	$ 1.7
Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Consent decree penalty				$ 1.0
Environmental loss contingencies					13.0	13.0
Increase (decrease) in the value of the asbestos liability reserve					1.0
Loss Contingency, Estimate of Possible Loss								$ 105.0
Cotter Corporation [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Number of Defendants | Defendant	16	13
Cotter Corporation [Member] | Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total cost of remediation to be shared by PRPs						60.0
DOJ potential settlement						$ 90.0
Sixty-Eighth Street Dump [Member] | Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency Number Of Parties Jointly And Severally Liable In Environmental Protection Agency Action | Principle_responsible_party						19
Minimum estimated clean-up costs for all potentially responsible parties					50.0	$ 50.0
Maximum estimated clean-up costs for all potentially responsible parties					64.0	64.0
Rossville ash site [Member] | Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss					$ 9.0	$ 9.0