Retirement Benefits - Additional (Details) (USD $)	Sep. 30, 2012	Mar. 11, 2012
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	$ 77,000,000
Expected non-qualified pension plan contributions	67,000,000
Expected other postretirement benefit plan contributions	318,000,000
Exelon Generation Co L L C [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	46,000,000
Expected non-qualified pension plan contributions	9,000,000
Expected other postretirement benefit plan contributions	131,000,000
Commonwealth Edison Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	9,000,000
Expected non-qualified pension plan contributions	14,000,000
Expected other postretirement benefit plan contributions	116,000,000
PECO Energy Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	13,000,000
Expected non-qualified pension plan contributions	1,000,000
Expected other postretirement benefit plan contributions	33,000,000
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Contributions [Abstract]
Expected non-qualified pension plan contributions	1,000,000
Expected other postretirement benefit plan contributions	13,000,000
Expected other postretirement benefit plan contributions for acquired entities		$ 4,000,000