Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
CRRs | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 366,000,000	$ 186,000,000
Fair Value, Liabilities	0	0
Tolling | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	5,000,000	4,000,000
Fair Value, Liabilities	1,175,000,000	1,005,000,000
Level 3 | CRRs | Market Simulation Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	3.50	2.95
Level 3 | CRRs | Market Simulation Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	7.10	7.78
Level 3 | CRRs | 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
Level 3 | CRRs | 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
Level 3 | CRRs | Market Simulation Model | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(9.86)	(13.90)
Level 3 | CRRs | Market Simulation Model | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	108.56	226.75
Level 3 | Tolling | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	16.00%	17.00%
Level 3 | Tolling | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	35.00%	36.00%
Level 3 | Tolling | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	21.00%	22.00%
Level 3 | 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)	38.00	35.00
Level 3 | 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)	63.90	84.10
Level 3 | Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	30.00%	29.00%
Expected prices (in dollars per unit)	$ 47.40	$ 55.40