Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 253,000,000	$ 221,000,000
Fair Value, Liabilities	1,284,000,000	1,012,000,000
Electricity Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	2,000,000	40,000,000
Fair Value, Liabilities	10,000,000	12,000,000
Electricity Options | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	28.00%	29.00%
Expected prices (in dollars per unit)	40.60	41.70
Electricity Options | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	45.00%	64.00%
Expected prices (in dollars per unit)	49.80	59.20
Electricity Options | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	39.00%	42.00%
Expected prices (in dollars per unit)	43.50	47.00
Electricity Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	24.00%	25.00%
Electricity Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	30.00%	36.00%
Electricity Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	28.00%	33.00%
Electricity Forwards | Discounted Cash Flow
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	0	2,000,000
Fair Value, Liabilities	4,000,000	4,000,000
Electricity Forwards | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	7.70	23.10
Electricity Forwards | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	40.80	44.90
Electricity Forwards | Discounted Cash Flow | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	31.40	31.10
Congestion Revenue Rights | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	245,000,000	186,000,000
Fair Value, Liabilities	0	0
Congestion Revenue Rights | Market Simulation Model | Load Forecast | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	7,603	7,597
Congestion Revenue Rights | Market Simulation Model | Load Forecast | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	24,896	26,612
Congestion Revenue Rights | Market Simulation Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(9.86)	(13.90)
Congestion Revenue Rights | Market Simulation Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	108.56	226.75
Congestion Revenue Rights | Market Simulation Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	3.50	2.95
Congestion Revenue Rights | Market Simulation Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	7.10	7.78
Gas Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets		0
Fair Value, Liabilities		2,000,000
Gas Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)		28.00%
Gas Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)		36.00%
Gas Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)		34.00%
Tolling | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	8,000,000	4,000,000
Fair Value, Liabilities	1,272,000,000	1,005,000,000
Tolling | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	25.00%	26.00%
Expected prices (in dollars per unit)	35.90	35.00
Tolling | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	45.00%	64.00%
Expected prices (in dollars per unit)	62.60	84.10
Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	29.00%	29.00%
Expected prices (in dollars per unit)	46.90	55.40
Tolling | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	16.00%	17.00%
Tolling | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	30.00%	36.00%
Tolling | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	19.00%	22.00%
Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(2,000,000)	(11,000,000)
Fair Value, Liabilities	$ (2,000,000)	$ (11,000,000)