Benefit Plans - Benefit Obligations and Accrued Costs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Changes in benefit obligations and the funded status
Accumulated benefit obligations	$ 2,854	$ 2,563
Accrued benefit costs
Noncurrent liabilities	(1,625)	(1,514)
PEC
Accrued benefit costs
Noncurrent liabilities	(687)	(723)
PEF
Accrued benefit costs
Noncurrent liabilities	(598)	(522)
Pension Benefits
Changes in benefit obligations and the funded status
Projected benefit obligation at January 1	2,609	2,422
Service cost	53	48	42
Interest cost	141	140	138
Settlements	(6)	0
Benefit payments	(129)	(129)
Plan amendment	0	1
Actuarial loss	238	127
Obligation at December 31	2,906	2,609	2,422
Fair value of plan assets at December 31	2,191	1,891	1,673
Funded status	(715)	(718)
Accrued benefit costs
Current liabilities	(10)	(10)
Noncurrent liabilities	(705)	(708)
Funded status	(715)	(718)
Recognized in accumulated other comprehensive loss
Net actuarial loss	34	90
Other, net	2	9
Recognized in regulatory assets, net
Net actuarial loss	1,139	824
Other, net	56	55
Total not yet recognized as a component of net periodic cost	1,231	978
Defined benefit plan, amounts expected to recognize as components of net periodic costs next year
Amortization of actuarial loss	91
Amortization of other, net	9
Weighted-average actuarial assumptions used in the calculation of year-end obligations
Discount rate	4.75%	5.65%
Rate of increase in future compensation
Bargaining	4.00%	4.50%
Supplementary plans	5.25%	5.25%
Other Postretirement Benefits
Changes in benefit obligations and the funded status
Projected benefit obligation at January 1	733	543
Service cost	11	16	7
Interest cost	41	45	31
Settlements	0	0
Benefit payments	(42)	(44)
Plan amendment	0	0
Actuarial loss	98	173
Obligation at December 31	841	733	543
Fair value of plan assets at December 31	37	33	55
Funded status	(804)	(700)
Accrued benefit costs
Current liabilities	(22)	(22)
Noncurrent liabilities	(782)	(678)
Funded status	(804)	(700)
Recognized in accumulated other comprehensive loss
Net actuarial loss	0	5
Other, net	0	1
Recognized in regulatory assets, net
Net actuarial loss	274	183
Other, net	3	9
Total not yet recognized as a component of net periodic cost	277	198
Defined benefit plan, amounts expected to recognize as components of net periodic costs next year
Amortization of actuarial loss	23
Amortization of other, net	$ 4
Weighted-average actuarial assumptions used in the calculation of year-end obligations
Discount rate	4.85%	5.75%
Rate of increase in future compensation
Initial medical cost trend rate for pre-Medicare Act benefits	8.75%	8.50%
Initial medical cost trend rate for post-Medicare Act benefits	8.75%	8.50%
Ultimate medical cost trend rate	5.00%	5.00%
Year ultimate medical cost trend rate is achieved	2020	2017