Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 35.0	$ 115.4	$ 89.8
Regulatory Assets, Noncurrent	1,293.4	1,251.8	1,282.8
Total Regulatory Assets	1,328.4	1,367.2	1,372.6
Regulatory Liabilities, Current	48.6	49.5	37.9
Regulatory Liabilities, Noncurrent	582.0	535.5	547.5
Total Regulatory Liabilities	630.6	585.0	585.4
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	8.7	84.0	63.0
Regulatory Assets, Noncurrent	613.5	588.0	634.1
Total Regulatory Assets	622.2	672.0	697.1
Regulatory Liabilities, Current	14.0	10.2	0.0
Regulatory Liabilities, Noncurrent	491.1	443.3	465.7
Total Regulatory Liabilities	505.1	453.5	465.7
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	13.2	19.2	14.7
Regulatory Assets, Noncurrent	659.6	642.0	626.0
Total Regulatory Assets	672.8	661.2	640.7
Regulatory Liabilities, Current	29.3	33.8	32.2
Regulatory Liabilities, Noncurrent	25.8	28.3	19.6
Total Regulatory Liabilities	55.1	62.1	51.8
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	1.2	28.9	58.0
Regulatory Assets, Noncurrent	0.0	1.2	0.0
Regulatory Liabilities, Current	44.0	42.3	33.3
Regulatory Liabilities, Noncurrent	44.8	17.2	83.6
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0.0	28.1	57.6
Regulatory Assets, Noncurrent	0.0	1.2	0.0
Regulatory Liabilities, Current	14.0	10.2	0.0
Regulatory Liabilities, Noncurrent	44.8	17.2	83.6
Unamortized Purchased Gas Adjustments [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Current	29.3	30.9	32.2
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	33.8	86.5	31.8
Regulatory Assets, Noncurrent	151.1	133.2	173.5
Regulatory Liabilities, Current	4.6	7.2	4.6
Regulatory Liabilities, Noncurrent	34.1	37.6	18.7
Other [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	8.7	55.9	5.4
Regulatory Assets, Noncurrent	138.7	119.6	159.7
Regulatory Liabilities, Noncurrent	30.2	33.2	14.2
Other [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	13.2	19.2	14.7
Regulatory Assets, Noncurrent	0.5	0.8	0.9
Regulatory Liabilities, Current	0.0	2.9	0.0
Regulatory Liabilities, Noncurrent	2.8	3.2	3.2
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	236.4	237.5	243.2
Regulatory Liabilities, Noncurrent	261.1	232.9	192.3
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	172.5	166.5	177.5
Regulatory Liabilities, Noncurrent	227.2	196.6	165.7
Pension and Postretirement Benefit Costs [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	61.8	68.2	62.3
Regulatory Liabilities, Noncurrent	23.0	25.1	16.4
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	687.2	668.2	658.0
Regulatory Liabilities, Noncurrent	138.3	133.6	135.4
Cost of Removal [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	91.4	97.0	97.0
Regulatory Liabilities, Noncurrent	96.9	94.5	97.1
Cost of Removal [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	595.7	571.2	561.0
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	155.0	150.7	149.0
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	147.2	142.7	140.8
Future Income Taxes Due from Customers [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	1.6	1.8	1.8
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	63.7	61.0	59.1
Energy Efficiency [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	63.7	61.0	59.1
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	103.7	114.2	117.5
Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 92.0	$ 101.8	$ 105.1

Schedule of regulatory assets and liabilities.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
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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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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
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