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Note 1 - Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member] |  |  |
Natural gas purchases from Spire Marketing | $ 57.4 | $ 86.3 | $ 92.5
Transaction amount | 0.4 | 0.0 | 1.1
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member] |  |  |
Natural gas purchases from Spire Marketing | 4.7 | 3.2 | 10.4
Transaction amount | 0.0 | 0.5 | 0.1
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member] |  |  |
Transaction amount | 32.0 | 32.0 | 32.0
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Alabama Inc [Member] |  |  |
Transaction amount | 0.0 | 0.0 | 0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member] |  |  |
Transaction amount | 0.7 | 0.0 | 0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Alabama Inc [Member] |  |  |
Transaction amount | 0.0 | 0.0 | 0.0
Unregulated Operation [Member] | Spire N G L Inc [Member] | Spire Missouri [Member] |  |  |
Transaction amount | 0.0 | 0.0 | 0.5
Unregulated Operation [Member] | Spire N G L Inc [Member] | Spire Alabama Inc [Member] |  |  |
Transaction amount | $ 0.0 | $ 0.0 | $ 0.0

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- Definition Amount of transactions with related party during the financial reporting period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 850 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483326/850-10-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483326/850-10-50-3 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |