Income Taxes - Federal and State Loss Carryforwards, Contribution Carryforwards, and Various Tax Credit Carryforwards (Details) $ in Millions	Sep. 30, 2021 USD ($)
Spire
Income Tax Contingency [Line Items]
Federal and state loss carryforwards	$ 1,079.5
Contribution carryforwards	16.4
Tax credit carryforwards	4.5
Spire Missouri
Income Tax Contingency [Line Items]
Federal and state loss carryforwards	467.3
Contribution carryforwards	12.2
Tax credit carryforwards	3.4
Spire Alabama
Income Tax Contingency [Line Items]
Federal and state loss carryforwards	517.5
Contribution carryforwards	$ 0.5

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible charitable contribution carryforwards.

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

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