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Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Statement of Comprehensive Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Derivative [Line Items] |  |  |
Net hedging gain (loss) arising during the period | $ (8.9) | $ (46.4) | $ 3.9
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships |  |  |
Derivative [Line Items] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | (3.2) | 1.3 | 1.5
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) |  |  |
Derivative [Line Items] |  |  |
Net hedging gain (loss) arising during the period | (8.9) | (46.4) | 3.9
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and Heating Oil Contracts |  |  |
Derivative [Line Items] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | 0.0 | 0.0 | 0.1
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) | Gasoline and Heating Oil Contracts |  |  |
Derivative [Line Items] |  |  |
Net hedging gain (loss) arising during the period | 0.0 | 0.0 | 0.0
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and Heating Oil Contracts |  |  |
Derivative [Line Items] |  |  |
Effective portion of gain (loss) reclassified from AOCI to income | $ 0.0 | $ 0.0 | $ 0.1

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624177-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
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- Definition Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558989&loc=d3e80720-113993 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580 + Details
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