Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 4.8	$ 6.0	$ 8.3	$ 8.2
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	0.0	9.0	0.0	13.9
Unregulated Operation [Member] \| Spire MoGas Pipeline [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	9.9	9.7	19.9	19.6
Unregulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 0.1	$ 0.1	$ 0.1	$ 0.1

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 2: 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 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
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-Publisher FASB
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
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-Name Accounting Standards Codification
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-URI https://asc.fasb.org/1943274/2147481990/310-10-45-13

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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-Section S99
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