Accrued pension assets added to prepaid pension asset.

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Change in net deferred gas costs and average inventory costs.

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Provision for regulatory liability.

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- Definition

Public utilities approved percentage of excess natural gas supply and capacity to customers.

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Public utilities approved percentage of excess natural gas supply and capacity to retain.

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- Definition

Regulatory liability with reduction in revenue.

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Remaining Recovery Period of Certain Regulatory Assets for which No Return on Investment During Recovery Period is Provided

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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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.

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Percentage of public utility's requested equity to capital, including debt and equity.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested return on equity.

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- Definition

The amount of regulated gas operating revenues recognized during the period.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of operating expense for purchased gas and petroleum of regulated operation.

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Regulatory Matters - Additional Information (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Aug. 05, 2021	Dec. 11, 2020	Feb. 28, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020	Feb. 09, 2021
Public Utilities, General Disclosures [Line Items]
Remaining recovery period for certain regulatory assets for which no return on investment during recovery period is provided (in years)							20 years
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided (in years)							15 years
Regulatory assets, current				$ 65.1		$ 70.2	$ 65.1	$ 70.2	$ 69.5
Natural gas			$ 195.8	96.9		72.9	897.2	631.9
Incremental gross revenue			3.2
Spire STL Pipeline LLC \| Subsequent Event
Public Utilities, General Disclosures [Line Items]
Impairment of assets	$ 270.0
Spire Missouri
Public Utilities, General Disclosures [Line Items]
Regulatory assets, current				36.6		29.5	36.6	29.5	32.1
Natural gas				55.6		$ 68.9	754.6	$ 478.3
Incremental gross revenue			100.0
Regulatory liability with a reduction in revenue			$ 25.0
Approved percentage of excess natural gas supply and capacity retain			25.00%
Approved percentage of excess natural gas supply and capacity to customers			75.00%
Change in net deferred gas costs and average inventory costs					$ 110.0
MoPSC
Public Utilities, General Disclosures [Line Items]
Regulatory assets, current				$ 2.9			2.9		3.8
Approved rate increase (decrease) amount		$ 64.2							$ 2,780.0
Requested rate increase		$ 111.5
Common equity ratio		54.25%
Requested return rate on equity		9.95%
Infrastructure System Replacement Surcharge \| MoPSC
Public Utilities, General Disclosures [Line Items]
Accrued pension assets added to prepaid pension asset										$ 9.0
Requested rate increase		$ 47.3
Infrastructure System Replacement Surcharge \| MoPSC \| Spire Missouri
Public Utilities, General Disclosures [Line Items]
Provision for regulatory liability							4.8
After-tax impact on loss contingency provision reducing net income							$ 3.7