Commitments and Contingencies Commitments and Contingencies - Commitments Table (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Contractual Obligation [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 6
Operating Leases, Future Minimum Payments, Due in Two Years	5
Operating Leases, Future Minimum Payments, Due in Three Years	4
Operating Leases, Future Minimum Payments, Due in Four Years	3
Operating Leases, Future Minimum Payments, Due in Five Years	2
Operating Leases, Future Minimum Payments, Due Thereafter	44
Operating Leases, Future Minimum Payments Due	64
Contractual Obligation, Due in Next Twelve Months	1,394
Contractual Obligation, Due in Second Year	1,040
Contractual Obligation, Due in Third Year	754
Contractual Obligation, Due in Fourth Year	597
Contractual Obligation, Due in Fifth Year	543
Contractual Obligation, Due after Fifth Year	3,274
Contractual Obligation	7,602
Rent expense for power purchase agreements meeting definition of operating lease	19	28	26
Maximum percentage of energy sources for which a share of operating costs and debt service is required	5.00%	5.00%	5.00%
Rent expense	14	18	15
Purchased electricity contracts [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	178
Purchase Obligation, Due in Second Year	112
Purchase Obligation, Due in Third Year	113
Purchase Obligation, Due in Fourth Year	94
Purchase Obligation, Due in Fifth Year	69
Purchase Obligation, Due after Fifth Year	450
Purchase Obligation	1,016
Fuel contracts [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	666
Purchase Obligation, Due in Second Year	646
Purchase Obligation, Due in Third Year	525
Purchase Obligation, Due in Fourth Year	411
Purchase Obligation, Due in Fifth Year	389
Purchase Obligation, Due after Fifth Year	1,978
Purchase Obligation	4,615
Construction commitments [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	408
Purchase Obligation, Due in Second Year	158
Purchase Obligation, Due in Third Year	25
Purchase Obligation, Due in Fourth Year	13
Purchase Obligation, Due in Fifth Year	10
Purchase Obligation, Due after Fifth Year	60
Purchase Obligation	674
Transmission [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	105
Purchase Obligation, Due in Second Year	97
Purchase Obligation, Due in Third Year	75
Purchase Obligation, Due in Fourth Year	68
Purchase Obligation, Due in Fifth Year	61
Purchase Obligation, Due after Fifth Year	671
Purchase Obligation	1,077
Maintenance, service and other contracts [Member]
Contractual Obligation [Line Items]
Purchase Obligation, Due in Next Twelve Months	31
Purchase Obligation, Due in Second Year	22
Purchase Obligation, Due in Third Year	12
Purchase Obligation, Due in Fourth Year	8
Purchase Obligation, Due in Fifth Year	12
Purchase Obligation, Due after Fifth Year	71
Purchase Obligation	$ 156