Retirement Benefits - Defined Contribution Savings Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended	6 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014 Exelon Generation Co L L C [Member]		Sep. 30, 2013 Exelon Generation Co L L C [Member]		Sep. 30, 2014 Exelon Generation Co L L C [Member]		Sep. 30, 2013 Exelon Generation Co L L C [Member]		Sep. 30, 2014 Commonwealth Edison Co [Member]	Sep. 30, 2013 Commonwealth Edison Co [Member]	Sep. 30, 2014 Commonwealth Edison Co [Member]	Sep. 30, 2013 Commonwealth Edison Co [Member]	Sep. 30, 2014 PECO Energy Co [Member]	Sep. 30, 2013 PECO Energy Co [Member]	Sep. 30, 2014 PECO Energy Co [Member]	Sep. 30, 2013 PECO Energy Co [Member]	Sep. 30, 2014 Baltimore Gas and Electric Company [Member]	Sep. 30, 2013 Baltimore Gas and Electric Company [Member]	Sep. 30, 2014 Baltimore Gas and Electric Company [Member]	Sep. 30, 2013 Baltimore Gas and Electric Company [Member]	Sep. 30, 2014 Business Services Company [Member]		Sep. 30, 2013 Business Services Company [Member]		Sep. 30, 2014 Business Services Company [Member]		Sep. 30, 2013 Business Services Company [Member]		Sep. 30, 2014 CENG [Member]	Sep. 30, 2014 CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	$ 34	[1]	$ 18	[1]	$ 82	[1]	$ 61	[1]	$ 17	[1]	$ 8	[1]	$ 41	[1]	$ 29	[1]	$ 8	$ 6	$ 20	$ 16	$ 2	$ 2	$ 6	$ 6	$ 3	$ 1	$ 7	$ 5	$ 4	[2]	$ 1	[2]	$ 8	[2]	$ 5	[2]	$ 1	$ 1

[1] Includes $1 million related to CENG for the three months ended September 30, 2014 and for the period from April 1, 2014 to September, 30 2014. CENG is not included in the 2013 amounts.

[2] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These costs are not included in the Generation, ComEd, PECO or BGE amounts above.