Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	203	221
Fair Value, Liabilities	1,085	1,012
Electricity Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	35	40
Fair Value, Liabilities	10	12
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)	29.00%	36.00%
Electricity Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	27.00%	33.00%
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)	47.30	41.70
Electricity Options | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	58.00%	64.00%
Expected prices (in dollars per unit)	60.70	59.20
Electricity Options | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	40.00%	42.00%
Expected prices (in dollars per unit)	52.90	47.00
Electricity Forwards | Discounted Cash Flow
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	4	2
Fair Value, Liabilities	1	4
Electricity Forwards | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	11.00	23.10
Electricity Forwards | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	52.00	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)	40.10	31.10
CRRs | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	170	186
Fair Value, Liabilities	0	0
CRRs | Market Simulation Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	2.95	2.95
CRRs | Market Simulation Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	7.78	7.78
CRRs | Market Simulation Model | Load Forecast | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	7,597	7,597
CRRs | Market Simulation Model | Load Forecast | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	26,612	26,612
CRRs | Market Simulation Model | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(13.90)	(13.90)
CRRs | Market Simulation Model | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	226.75	226.75
Gas Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets		0
Fair Value, Liabilities		2
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	4	4
Fair Value, Liabilities	1,084	1,005
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)	29.00%	36.00%
Tolling | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	20.00%	22.00%
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)	42.40	35.00
Tolling | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	58.00%	64.00%
Expected prices (in dollars per unit)	76.50	84.10
Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	28.00%	29.00%
Expected prices (in dollars per unit)	54.90	55.40
Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(10)	(11)
Fair Value, Liabilities	(10)	(11)