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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $) | 0 Months Ended | 3 Months Ended
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Apr. 16, 2013 | Dec. 31, 2012 | Dec. 31, 2012 Private Partnerships | Dec. 31, 2011 Private Partnerships | Dec. 31, 2012 Commodity Contract Derivatives | Dec. 31, 2011 Commodity Contract Derivatives | Dec. 31, 2012 Interest Rate Swaption | Dec. 31, 2011 Interest Rate Swaption | Dec. 31, 2012 Minimum tons-per-year | Dec. 31, 2012 Maximum tons-per-year
Fair Value Measurements |  |  |  |  |  |  |  |  |  |  |  |
Balances at beginning of period |  |  | $ 53,000,000 | $ 22,000,000 | $ (267,000,000) | $ 239,000,000 | $ 0 | $ (1,077,000,000) |  |  |  |
Purchases |  |  | 12,000,000 | 6,000,000 | 0 | 0 | 0 | 0 |  |  |  |
Issuances |  |  | 0 | 0 | 0 | 0 | 0 | 0 |  |  |  |
Sales |  |  | (1,000,000) | (1,000,000) | 0 | 0 | 0 | 0 |  |  |  |
Settlements |  |  | 0 | 0 | 0 | 0 | 0 | 0 |  |  |  |
Net realized (gains) losses recognized in income |  | 0 |  |  |  |  |  |  |  |  |  |
Net unrealized gains (losses) deferred as regulatory assets and liabilities |  |  | 2,000,000 | 1,000,000 | 43,000,000 | (235,000,000) | 0 | (51,000,000) |  |  |  |
Balances at end of period |  |  | 66,000,000 | 28,000,000 | (224,000,000) | 4,000,000 | 0 | (1,128,000,000) |  |  |  |
Proceeds from derivative instrument financing | (60,000,000) |  |  |  |  |  |  |  |  |  |  |
Assets |  |  |  |  |  |  |  |  |  |  |  |
Fair value inputs, counterparty credit risk |  |  |  |  |  |  |  |  | 28.60% | [1] | 28.60% | [1]
Fair value measurements tons per year |  |  |  |  |  |  |  |  | 1,000,000,000 |  | 1,100,000,000 |
Price per ton |  |  |  |  |  |  |  |  | 13.75 |  | 86.83 |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |
Fair value measurements tons per year |  |  |  |  |  |  |  |  | 1,000,000,000 |  | 1,100,000,000 |
Price per ton |  |  |  |  |  |  |  |  | $ 13.75 |  | $ 86.83 |
[1] | Applies to only one contract.

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- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are deferred as regulatory assets and liabilities; the fair value of which assets or liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + References No definition available. + Details
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Balance Type: | credit
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- Definition The fair value measurement of tons per year used to determine fair value + References No definition available. + Details
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Balance Type: | na
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- Definition Issues that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Purchases that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Sales that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Settlements that have taken place during the period in relation to assets or liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition This element represents an asset or liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References No definition available. + Details
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- Definition The price per ton used for the determination of fair value. + References No definition available. + Details
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- Details
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- Definition The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 + Details
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- Definition Risk that one party to the contract will default expressed as a percentage, used as an input to measure fair value. + 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 (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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- Definition The net cash outflow or inflow from derivative instruments during the period, which are classified as financing activities, excluding those designated as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3505-108585 + Details
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