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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Designated as Hedging Instrument | Cash Flow Hedging |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | $ 0.0 | $ (1.1) | $ (4.4)
Ineffective portion of gain (loss) on derivatives recognized in income | 0.5 | 0.3 | (0.4)
Designated as Hedging Instrument | Cash Flow Hedging | Other Comprehensive Income (Loss) |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) recognized in OCI on derivatives | 11.5 | (4.0) | (5.5)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | (0.3) | (0.6) | (3.5)
Ineffective portion of gain (loss) on derivatives recognized in income | 0.0 | 0.2 | (0.5)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Other Comprehensive Income (Loss) |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) recognized in OCI on derivatives | 0.0 | (0.6) | (4.3)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Gas Marketing Operating Revenues |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | (0.4) | 4.3 | 1.7
Ineffective portion of gain (loss) on derivatives recognized in income | 0.0 | 0.1 | 0.0
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Gas Marketing Operating Expenses |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | 0.1 | (4.9) | (5.2)
Ineffective portion of gain (loss) on derivatives recognized in income | 0.0 | 0.1 | (0.5)
Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Other Comprehensive Income (Loss) |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) recognized in OCI on derivatives | 0.1 | 0.0 | (1.2)
Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | 0.2 | (0.5) | (0.9)
Ineffective portion of gain (loss) on derivatives recognized in income | 0.0 | 0.1 | 0.1
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swaps | Other Comprehensive Income (Loss) |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) recognized in OCI on derivatives | 11.4 | (3.4) | 0.0
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swaps | Interest Expense |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | 0.1 | 0.0 | 0.0
Ineffective portion of gain (loss) on derivatives recognized in income | 0.5 | 0.0 | 0.0
Not Designated as Hedging Instrument |  |  |
Derivatives Not Designated as Hedging Instruments [Abstract] |  |  |
Gain (loss) recognized in income on derivatives | (3.7) | 10.6 | (11.1)
Not Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Income and (Income Deductions) - Net |  |  |
Derivatives Not Designated as Hedging Instruments [Abstract] |  |  |
Gain (loss) recognized in income on derivatives | 0.0 | 0.0 | (0.2)
Not Designated as Hedging Instrument | Natural gas commodity contracts | Gas Marketing Operating Revenues |  |  |
Derivatives Not Designated as Hedging Instruments [Abstract] |  |  |
Gain (loss) recognized in income on derivatives | 0.7 | 12.3 | (1.3)
Not Designated as Hedging Instrument | NYMEX / ICE natural gas contracts | Gas Marketing Operating Revenues |  |  |
Derivatives Not Designated as Hedging Instruments [Abstract] |  |  |
Gain (loss) recognized in income on derivatives | (4.4) | (1.7) | (9.6)
Spire Missouri | Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Other Comprehensive Income (Loss) |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) recognized in OCI on derivatives | 0.1 | 0.0 | (1.2)
Spire Missouri | Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses |  |  |
Derivatives in Cash Flow Hedging Relationships [Abstract] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | 0.2 | (0.5) | (0.9)
Ineffective portion of gain (loss) on derivatives recognized in income | 0.0 | 0.1 | 0.1
Spire Missouri | Not Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Income and (Income Deductions) - Net |  |  |
Derivatives Not Designated as Hedging Instruments [Abstract] |  |  |
Gain (loss) recognized in income on derivatives | $ 0.0 | $ 0.0 | $ (0.2)

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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying 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 (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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- Definition The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness. + 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 (d) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1),(2) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + 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 (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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