Retirement Benefits - Additional (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	$ 255
Expected non-qualified pension plan contributions	79
Expected other postretirement benefit plan contributions	292
Exelon Generation Co L L C [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	113
Expected non-qualified pension plan contributions	6
Expected other postretirement benefit plan contributions	117
Commonwealth Edison Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	116
Expected non-qualified pension plan contributions	1
Expected other postretirement benefit plan contributions	114
PECO Energy Co [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	11
Expected non-qualified pension plan contributions	1
Expected other postretirement benefit plan contributions	22
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Contributions [Abstract]
Expected qualified pension plan contributions	0
Expected non-qualified pension plan contributions	2
Expected other postretirement benefit plan contributions	$ 18