Retirement Benefits - Defined Contribution Savings Plans (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	$ 38	$ 19	$ 60	$ 48
CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	3	1	4
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	20	10	33	24
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		8	5	13	12
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		3	2	4	4
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions		3	1	5	4
Business Services Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[2]	$ 4	$ 1	$ 5	$ 4

[1] Includes $3 million and $4 million, respectively, related to CENG for the three and six months ended June 30, 2015. For the period of April 1, 2014 to June 30, 2014, amounts include $1 million related to CENG.

[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.