Commitments and Contingencies - Environmental Issues Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	1 Months Ended	0 Months Ended
	Mar. 31, 2015	Oct. 31, 2007	Apr. 12, 2012 Defendant	Feb. 28, 2012 Defendant	Sep. 30, 2014
Accrual For Environmental Loss Contingencies [Line Items]
Litigation Settlement, Amount	$ 14
Commonwealth Edison Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites	42
Approved clean-up	17
Sites under study/remediation	25
PECO Energy Co [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites	26
Approved clean-up	16
Sites under study/remediation	10
Baltimore Gas and Electric Company [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Total number of MGP sites	13
Sites under study/remediation	2
Maximum estimated clean-up costs for all potentially responsible parties	1.7
Exelon Generation Co L L C [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Consent decree penalty		1
Environmental loss contingencies	13
Loss Contingency, Estimate of Possible Loss					105
Cotter Corporation [Member]
Accrual For Environmental Loss Contingencies [Line Items]
Loss Contingency, Number of Defendants			14	15
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	50
DOJ potential settlement	90
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	19
Minimum estimated clean-up costs for all potentially responsible parties	50
Maximum estimated clean-up costs for all potentially responsible parties	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	$ 10