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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)

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- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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