Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 1,004	$ 705
Fair Value, Liabilities	69	118
Level 3
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	26	52
Fair Value, Liabilities	5	1
Level 3 | Electricity Congestion Contracts
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	42	71
Fair Value, Liabilities	17	20
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	(34.77)	(8.93)
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	18.70	18.03
Level 3 | Electricity Congestion Contracts | Latest Auction Pricing | Congestion Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	0.10	0.19
Level 3 | Electricity Power Contracts
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	3	2
Fair Value, Liabilities	7	2
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	30.43	22.54
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	58.65	48.85
Level 3 | Electricity Power Contracts | Discounted Cash Flow | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value inputs, price level	37.41	39.62
Level 3 | Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(19)	(21)
Fair Value, Liabilities	$ (19)	$ (21)