Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 MW		Dec. 31, 2013 MW
Congestion revenue rights | Market simulation model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	317		366
Fair Value, Liabilities	0		0
Tolling | Option model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	4		5
Fair Value, Liabilities	1,207		1,175
Level 3 | Congestion revenue rights | Market simulation model | Load forecast | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	7,630		7,603
Level 3 | Congestion revenue rights | Market simulation model | Load forecast | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	25,431		24,896
Level 3 | Congestion revenue rights | Market simulation model | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	1.65	[1]	(9.86)	[1]
Level 3 | Congestion revenue rights | Market simulation model | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	109.95	[1]	108.56	[1]
Level 3 | Congestion revenue rights | Market simulation model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	3.65	[2]	3.50	[2]
Level 3 | Congestion revenue rights | Market simulation model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	6.53	[2]	7.10	[2]
Level 3 | Tolling | Option model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	13.00%		16.00%
Level 3 | Tolling | Option model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	53.00%		35.00%
Level 3 | Tolling | Option model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	20.00%		21.00%
Level 3 | Tolling | Option model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	25.00%		25.00%
Expected prices (in dollars per unit)	30.60		38.00
Level 3 | Tolling | Option model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	42.00%		45.00%
Expected prices (in dollars per unit)	61.40		63.90
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%		30.00%
Expected prices (in dollars per unit)	44.60		47.40

[1] Prices are in dollars per megawatt-hour.

[2] Prices are in dollars per million British thermal units.