Retirement Benefits - Additional (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	$ 255
Expected non-qualified pension plan contributions	82
Expected other postretirement benefit plan contributions	276
Exelon Generation Co L L C [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	113
Expected non-qualified pension plan contributions	7
Expected other postretirement benefit plan contributions	108
Commonwealth Edison Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	115
Expected non-qualified pension plan contributions	1
Expected other postretirement benefit plan contributions	112
PECO Energy Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	11
Expected non-qualified pension plan contributions	0
Expected other postretirement benefit plan contributions	21
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Contributions [Abstract]
Expected non-qualified pension plan contributions	2
Expected other postretirement benefit plan contributions	$ 17