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Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Sep. 30, 2020 | Mar. 31, 2020
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Quoted Prices in Active Markets (Level 1) |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | $ 104.0 | $ 4.1 | $ 108.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents |  |  | 5.1
Quoted Prices in Active Markets (Level 1) | Spire Alabama |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | 18.2 |  | 0.1
Significant Observable Inputs (Level 2) |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Notes payable | 653.5 | 648.0 | 560.6
Long-term debt, including current portion | 3,090.9 | 2,908.6 | 2,671.0
Significant Observable Inputs (Level 2) | Spire Missouri |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Notes payable | 250.0 |  | 185.1
Notes payable – associated companies | 369.4 | 301.2 | 39.6
Long-term debt, including current portion | 1,254.2 | 1,313.5 | 1,231.2
Significant Observable Inputs (Level 2) | Spire Alabama |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Notes payable |  |  | 50.0
Notes payable – associated companies |  | 121.3 | 33.0
Notes receivable, associated companies | 20.0 |  |
Long-term debt, including current portion | 682.3 | 576.9 | 535.1
Carrying Amount |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | 104.0 | 4.1 | 108.4
Notes payable | 653.5 | 648.0 | 560.6
Long-term debt, including current portion | 2,803.3 | 2,484.1 | 2,490.2
Carrying Amount | Spire Missouri |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents |  |  | 5.1
Notes payable | 250.0 |  | 185.1
Notes payable – associated companies | 369.4 | 301.2 | 39.6
Long-term debt, including current portion | 1,092.4 | 1,092.0 | 1,098.7
Carrying Amount | Spire Alabama |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | 18.2 |  | 0.1
Notes payable |  |  | 50.0
Notes payable – associated companies |  | 121.3 | 33.0
Notes receivable, associated companies | 20.0 |  |
Long-term debt, including current portion | 621.1 | 471.8 | 471.7
Fair Value |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | 104.0 | 4.1 | 108.4
Notes payable | 653.5 | 648.0 | 560.6
Long-term debt, including current portion | 3,090.9 | 2,908.6 | 2,671.0
Fair Value | Spire Missouri |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents |  |  | 5.1
Notes payable | 250.0 |  | 185.1
Notes payable – associated companies | 369.4 | 301.2 | 39.6
Long-term debt, including current portion | 1,254.2 | 1,313.5 | 1,231.2
Fair Value | Spire Alabama |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Cash and cash equivalents | 18.2 |  | 0.1
Notes payable |  |  | 50.0
Notes payable – associated companies |  | 121.3 | 33.0
Notes receivable, associated companies | 20.0 |  |
Long-term debt, including current portion | $ 682.3 | $ 576.9 | $ 535.1

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- Definition Notes receivable 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 Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + 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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