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Note 8 - Fair Value of Financial Instruments - Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Reported Value Measurement [Member] |  |
Cash and cash equivalents | $ 5.6 | $ 6.5
Notes payable | 955.5 | 1,037.5
Long-term debt, including current portion | 3,710.6 | 3,239.7
Reported Value Measurement [Member] | Spire Missouri [Member] |  |
Cash and cash equivalents | 0.8 |
Long-term debt, including current portion | 1,785.4 | 1,637.7
Notes payable - associated companies | 540.6 | 445.3
Reported Value Measurement [Member] | Spire Alabama Inc [Member] |  |
Cash and cash equivalents | 1.2 | 2.4
Long-term debt, including current portion | 745.9 | 571.5
Notes payable - associated companies | 124.1 | 260.9
Estimate of Fair Value Measurement [Member] |  |
Cash and cash equivalents | 5.6 | 6.5
Notes payable | 955.5 | 1,037.5
Long-term debt, including current portion | 3,270.2 | 2,851.8
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member] |  |
Cash and cash equivalents | 0.8 |
Long-term debt, including current portion | 1,592.4 | 1,473.9
Notes payable - associated companies | 540.6 | 445.3
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member] |  |
Cash and cash equivalents | 1.2 | 2.4
Long-term debt, including current portion | 648.0 | 485.0
Notes payable - associated companies | 124.1 | 260.9
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Cash and cash equivalents | 5.6 | 6.5
Notes payable | 0.0 | 0.0
Long-term debt, including current portion | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |
Cash and cash equivalents | 0.8 |
Long-term debt, including current portion | 0.0 | 0.0
Notes payable - associated companies | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |
Cash and cash equivalents | 1.2 | 2.4
Long-term debt, including current portion | 0.0 | 0.0
Notes payable - associated companies | 0.0 | 0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Cash and cash equivalents | 0.0 | 0.0
Notes payable | 955.5 | 1,037.5
Long-term debt, including current portion | 3,270.2 | 2,851.8
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |
Cash and cash equivalents | 0.0 |
Long-term debt, including current portion | 1,592.4 | 1,473.9
Notes payable - associated companies | 540.6 | 445.3
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |
Cash and cash equivalents | 0.0 | 0.0
Long-term debt, including current portion | 648.0 | 485.0
Notes payable - associated companies | $ 124.1 | $ 260.9

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- Definition Note payable related parties fair value. + References No definition available. + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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- Definition Fair value portion of notes payable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20,22) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 + Details
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