Retirement Benefits - Defined Contribution Savings Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	$ 51	$ 34	$ 111	$ 82
CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	4	1	8
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	27	17	60	41
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		10	8	23	20
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		3	2	7	6
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		5	3	10	7
Business Services Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[2]	$ 6	$ 4	$ 11	$ 8

[1] Includes $4 million and $8 million, respectively, related to CENG for the three and nine months ended September 30, 2015. 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.

[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.