Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Costs	$ 54	[1]	$ 87	[1]	$ 193	[1]	$ 259	[1]
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Costs	33		77		129		231
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Costs	7		11		28		32
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Costs	17		14		50		41
Business Services Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Costs	$ 11	[2]	$ 24	[2]	$ 37	[2]	$ 58	[2]

[1] For the three months ended September 30, 2014, the cost for pension benefits and other postretirement benefits related to CENG were $2 million and $3 million, respectively. For the period April 1, 2014 to September 30, 2014, the cost for pension benefits and other postretirement benefits related to CENG were $5 million and $6 million, respectively. CENG is not included in the 2013 amounts.

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