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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions | 12 Months Ended
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Sep. 30, 2015 USD ($) tons-per-year | Sep. 30, 2014 USD ($) Contracts tons-per-year
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Commodity contract derivatives | $ 1,000,000 | $ 1,000,000
Commodity contract derivatives | $ 98,000,000 | $ 97,000,000
Liabilities |  |
Number of contracts | Contracts |  | 2
Minimum |  |
Assets |  |
Fair value inputs, counterparty credit risk |  | 2.00%
Fair value measurements tons per year | tons-per-year | 0.8 | 1.0
Price per ton | $ 10.64 | $ 11.24
Liabilities |  |
Fair value measurements tons per year | tons-per-year | 0.8 | 1.0
Price per ton | $ 10.64 | $ 11.24
Maximum |  |
Assets |  |
Fair value inputs, counterparty credit risk |  | 5.00%
Fair value measurements tons per year | tons-per-year | 1.0 | 1.1
Price per ton | $ 103.41 | $ 67.07
Liabilities |  |
Fair value measurements tons per year | tons-per-year | 1.0 | 1.1
Price per ton | $ 103.41 | $ 67.07
Fair Value, Inputs, Level 3 |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Commodity contract derivatives | 1,000,000 | 1,000,000
Commodity contract derivatives | 98,000,000 | 97,000,000
Commodity Contract Derivatives |  |
Fair Value Measurements |  |
Balances at beginning of period | (96,000,000) | (140,000,000)
Purchases | 0 | 0
Issuances | 0 | 0
Sales | 0 | 0
Settlements | 0 | 33,000,000
Net unrealized gains (losses) deferred as regulatory assets and liabilities | (1,000,000) | 11,000,000
Balances at end of period | $ (97,000,000) | $ (96,000,000)

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- Definition The fair value as of the balance sheet date of firm obligations in physical commodity contracts (for example, agricultural products, metals and energy). + References No definition available. + Details
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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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- Definition [Line Items] for Schedule of the inputs used in the fair value measurement of assets and liabilities. This disclosure may include, but is not limited to, the fair value of the asset or liability, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs. + References No definition available. + Details
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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 fair value measurement of tons per year used to determine fair value + References No definition available. + Details
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- Definition The number of contracts the range applies to. + 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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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=36462937&loc=d3e19207-110258 + Details
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- Definition The fair value as of the balance sheet date of firm holdings in physical commodity contracts (for example, agricultural products, metals and energy). Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings. + References No definition available. + Details
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