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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES Narrative (Details) (USD $) |  |  |  | 12 Months Ended |  | 0 Months Ended |
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Sep. 30, 2014 MMBTU | Aug. 19, 2014 | Sep. 30, 2013 | Sep. 30, 2014 Laclede Gas MMBTU gal | Sep. 30, 2013 Laclede Gas | Aug. 06, 2014 Designated as Hedging Instrument Cash Flow Hedging Interest Rate Swaps | Jul. 08, 2014 Designated as Hedging Instrument Cash Flow Hedging Interest Rate Swaps
Derivative [Line Items] |  |  |  |  |  |  |
Maximum percentage of normal volumes hedged |  |  |  | 70.00% |  |  |
Maximum number of months normal volumes can be hedged |  |  |  | 36 months |  |  |
Gasoline futures contracts notional amount (in gallons) |  |  |  | 1,800,000 |  |  |
Average price of future contracts (dollars per gallon) |  |  |  | 2.60 |  |  |
Notional amount of non-exchange traded natural gas commodity contracts (in MMBtu) | 33,500,000 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2015 | 32,100,000 |  |  |  |  |  |
Notional amount of non-exchange traded natural gas commodity contracts to be settled during fiscal 2016 | 1,400,000 |  |  |  |  |  |
Notional amount |  |  |  |  |  |  | $ 375,000,000
Loss on termination and settlement of interest rate swap |  |  |  |  |  | 19,000,000 |
Long-term debt issued amount |  | 625,000,000.0 |  |  |  |  |
MMBtu (millions) |  |  |  | 21,000,000 |  |  |
Cash margin receivables not offset with derivatives | $ 4,600,000 |  | $ 3,200,000 | $ 4,400,000 | $ 3,200,000 |  |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Aggregate volume outstanding of gasoline futures contracts expressed in gallons. + References No definition available. + Details
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- Definition Maximum length of time over which the utility is hedging its exposure to price risk associated with purchasing commodities on behalf of its customers. Time period expressed in number of months, which may or may not be designated as hedging instruments. + References No definition available. + Details
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- Definition Maximum percentage the utility is allowed to hedge its exposure to price risk associated with purchasing natural gas on behalf of its customers, which may or may not be designated as hedging instruments. + References No definition available. + Details
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- Definition Aggregate notional amount of natural gas derivative contracts not designated as normal purchases or normal sales with notional amounts expressed in nonmonetary units. For example the number of MMBtus specified in a natural gas forward purchase contract. + References No definition available. + Details
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- Definition Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in The Next Twelve Months + References No definition available. + Details
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- Definition Notional Amount of Non-Exchange Traded Natural Gas Commodity Contracts To Be Settled in Year Two + References No definition available. + Details
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- Definition Amount of decrease in the fair value of derivatives recognized in the income statement. + References Reference 1: 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 Reference 2: 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 + Details
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