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Note 9 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Derivative Assets | $ 77.3 | $ 116.2
Derivative Liabilities | 30.8 | 61.5
Fair Value, Recurring [Member] |  |
Effects of netting, asset | (20.1) | (153.6)
Derivative Assets | 135.3 | 164.6
Effects of netting, liability | (69.4) | (117.0)
Derivative Liabilities | 30.8 | 61.5
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative Asset Fair Value Gross Asset | 74.4 | 198.0
Derivative Liability Fair Value Gross Liability | 72.6 | 113.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative Asset Fair Value Gross Asset | 81.0 | 120.2
Derivative Liability Fair Value Gross Liability | 27.6 | 65.5
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Gas Utility [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Derivative Assets | 20.4 | 19.1
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Other Segments [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Derivative Assets | 37.6 | 29.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 20.4 | 19.1
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 37.6 | 29.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Gas Utility [Member] |  |
Effects of netting, asset | (0.1) |
Derivative Assets | 0.0 |
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 |
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.1 |
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 |
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Utility [Member] |  |
Effects of netting, asset | (6.3) | (57.8)
Derivative Assets | 0.0 | 0.0
Effects of netting, liability | (44.0) | (30.7)
Derivative Liabilities | 6.8 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member] |  |
Effects of netting, asset | (10.1) | (91.8)
Derivative Assets | 0.0 | 0.0
Effects of netting, liability | (21.8) | (82.3)
Derivative Liabilities | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 6.3 | 57.8
Derivative Liability Fair Value Gross Liability | 50.8 | 30.7
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 10.1 | 91.8
Derivative Liability Fair Value Gross Liability | 21.8 | 82.3
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member] |  |
Effects of netting, asset | (3.6) | (4.0)
Derivative Assets | 33.1 | 52.6
Effects of netting, liability | (3.6) | (4.0)
Derivative Liabilities | 24.0 | 61.5
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 36.7 | 56.6
Derivative Liability Fair Value Gross Liability | 27.6 | 65.5
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Other Segments [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Derivative Assets | 44.2 | 63.6
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 44.2 | 63.6
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Segments [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Spire Missouri [Member] |  |
Derivative Assets | 0.0 | 0.0
Derivative Liabilities | 6.8 | 0.0
Spire Missouri [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | (6.4) | (57.8)
Derivative Assets | 20.4 | 19.1
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative Asset Fair Value Gross Asset | 26.7 | 76.9
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.1 | 0.0
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Derivative Assets | 20.4 | 19.1
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative Asset Fair Value Gross Asset | 20.4 | 19.1
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | (0.1) |
Derivative Assets | 0.0 |
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 |
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.1 |
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 |
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | (6.3) | (57.8)
Derivative Assets | 0.0 | 0.0
Effects of netting, liability | (44.0) | (30.7)
Derivative Liabilities | 6.8 | 0.0
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative Asset Fair Value Gross Asset | 6.3 | 57.8
Derivative Liability Fair Value Gross Liability | 50.8 | 30.7
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | 0.0 | 0.0
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative Asset Fair Value Gross Asset | 0.0 | 0.0
Derivative Liability Fair Value Gross Liability | $ 0.0 | $ 0.0

X
- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 5C -Subparagraph (SX 210.12-13C(Column F)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480032/946-320-S99-5C Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 5B -Subparagraph (SX 210.12-13B(Column E)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480032/946-320-S99-5B Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 22 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483444/210-20-55-22 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483466/210-20-50-3 + Details
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- Definition Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 5 -SubTopic 10 -Topic 815 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480463/815-10-45-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -SubTopic 20 -Topic 210 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483466/210-20-50-3 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 55 -Paragraph 8 -SubTopic 20 -Topic 210 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483444/210-20-55-8 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 21 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483444/210-20-55-21 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. 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