Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014 MW	Dec. 31, 2013 MW
Congestion Revenue Rights | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	$ 345,000,000	$ 366,000,000
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	7,603
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	24,896
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)	(9.86)
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	108.56
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	3.50
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	7.10
Tolling | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	4,000,000	5,000,000
Fair Value, Liabilities	1,111,000,000	1,175,000,000
Tolling | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	37.50	38.00
Expected volatility rate (as a percent)	26.00%	25.00%
Tolling | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	70.80	63.90
Expected volatility rate (as a percent)	61.00%	45.00%
Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	$ 50.10	$ 47.40
Expected volatility rate (as a percent)	32.00%	30.00%
Tolling | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	13.00%	16.00%
Tolling | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	31.00%	35.00%
Tolling | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	18.00%	21.00%