Retirement Benefits - Additional (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Healthcare Reform Legislation Impact [Abstract]
After-tax charge to income tax expense as a result of health care reform legislation	$ 65.00
Excise tax on certain high-cost benefit plans	40.00%
Components Of Net Periodic Benefit Costs [Abstract]
Effect of federal subsidy on net periodic postretirement benefit costs under the Prescription Drug Act	(17.00)	28.00	38.00
Securities Lending Program [Abstract]
Fair value of securities on loan	17.00
Fair value of cash and non-cash collateral received for loan securities	17.00
Projected Benefit Obligation In Excess Of Plan Assets [Member]
Benefit Obligation and Plan Assets, and Funded Status [Abstract]
Funded status of the pension and other postretirement benefit obligations	80.00%
Accumulated Benefit Obligation In Excess Of Plan Assets [Member]
Benefit Obligation and Plan Assets, and Funded Status [Abstract]
Funded status of the pension and other postretirement benefit obligations	85.00%
Exelon Generation Co L L C [Member]
Healthcare Reform Legislation Impact [Abstract]
After-tax charge to income tax expense as a result of health care reform legislation	24.00
Commonwealth Edison Co [Member]
Healthcare Reform Legislation Impact [Abstract]
After-tax charge to income tax expense as a result of health care reform legislation	11.00
PECO Energy Co [Member]
Healthcare Reform Legislation Impact [Abstract]
After-tax charge to income tax expense as a result of health care reform legislation	9.00
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Contributions [Abstract]
Expected other postretirement benefit plan contributions	$ 4.00	$ 13.00	$ 17.00