Benefit Plans - Benefit Obligations and Accrued Costs (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Changes in benefit obligations and the funded status
Change in pension benefit obligation due to defined benefit plans remeasurement	$ 122
Increase in regulatory assets due to defined benefit plans remeasurement	128
Weighted-average actuarial assumptions used in the calculation of year-end obligations
Defined benefit plan, health care cost trend rate assumed for currrent year	8.75%
Ultimate medical cost trend rate	5.00%
Year ultimate medical cost trend rate is achieved	2020
PEC [Member]
Changes in benefit obligations and the funded status
Change in pension benefit obligation due to defined benefit plans remeasurement	40
Increase in regulatory assets due to defined benefit plans remeasurement	44
PEF [Member]
Changes in benefit obligations and the funded status
Change in pension benefit obligation due to defined benefit plans remeasurement	75
Increase in regulatory assets due to defined benefit plans remeasurement	$ 75