Income Taxes - Summary of Federal and State Loss and Contribution and Tax Credit Carryforwards (Details) $ in Millions	Sep. 30, 2025 USD ($)
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	$ 646.1
Tax credit carryforwards	18.9
Spire Missouri [Member]
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	9.7
Tax credit carryforwards	8.8
Spire Alabama Inc [Member]
Tax Credit Carryforward [Line Items]
Federal and state loss carryforwards	583.3
Tax credit carryforwards	$ 2.7

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-3

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