Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Sep. 30, 2025	Mar. 31, 2025
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 78.7	$ 78.3	$ 42.3
Regulatory Assets, Noncurrent	1,394.1	1,323.5	1,287.2
Total Regulatory Assets	1,472.8	1,401.8	1,329.5
Regulatory Liabilities, Current	12.6	39.4	51.6
Regulatory Liabilities, Noncurrent	806.0	578.0	637.0
Total Regulatory Liabilities	818.6	617.4	688.6
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	12.6	48.9	15.5
Regulatory Assets, Noncurrent	671.4	653.2	613.8
Total Regulatory Assets	684.0	702.1	629.3
Regulatory Liabilities, Current	4.6	13.9	12.0
Regulatory Liabilities, Noncurrent	537.3	481.3	546.1
Total Regulatory Liabilities	541.9	495.2	558.1
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	18.3	16.1	13.8
Regulatory Assets, Noncurrent	661.9	650.6	652.5
Total Regulatory Assets	680.2	666.7	666.3
Regulatory Liabilities, Current	2.2	20.3	33.9
Regulatory Liabilities, Noncurrent	28.4	29.2	26.4
Total Regulatory Liabilities	30.6	49.5	60.3
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	3.4	0.0	0.0
Regulatory Assets, Noncurrent	211.2	217.8	241.3
Regulatory Liabilities, Noncurrent	305.4	300.1	256.1
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	150.2	157.0	175.0
Regulatory Liabilities, Noncurrent	267.4	261.2	221.6
Pension and Postretirement Benefit Costs [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	57.1	59.1	63.9
Regulatory Liabilities, Noncurrent	25.7	26.5	23.6
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	11.8	16.5	0.3
Regulatory Assets, Noncurrent	0.0	9.0	0.0
Regulatory Liabilities, Current	4.6	34.9	46.3
Regulatory Liabilities, Noncurrent	61.6	6.5	100.2
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	3.0	14.9	0.0
Regulatory Assets, Noncurrent	0.0	9.0	0.0
Regulatory Liabilities, Current	4.6	13.9	12.0
Regulatory Liabilities, Noncurrent	61.6	6.5	100.2
Unamortized Purchased Gas Adjustments [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	6.1	0.0	0.0
Regulatory Liabilities, Current	0.0	20.2	33.4
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	63.5	61.8	42.0
Regulatory Assets, Noncurrent	252.8	189.2	148.7
Regulatory Liabilities, Current	8.0	4.5	5.3
Regulatory Liabilities, Noncurrent	31.7	32.9	35.1
Other [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	9.6	34.0	15.5
Regulatory Assets, Noncurrent	205.4	176.8	135.8
Regulatory Liabilities, Noncurrent	27.5	28.8	31.2
Other [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	12.2	16.1	13.8
Regulatory Assets, Noncurrent	0.5	0.3	0.5
Regulatory Liabilities, Current	2.2	0.1	0.5
Regulatory Liabilities, Noncurrent	2.7	2.7	2.8
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	708.1	688.3	679.9
Regulatory Liabilities, Noncurrent	275.8	138.2	138.5
Cost of Removal [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	101.3	98.8	93.5
Regulatory Liabilities, Noncurrent	98.5	96.0	97.8
Cost of Removal [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	602.9	589.6	586.4
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	155.1	154.2	153.7
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	147.6	146.6	145.9
Future Income Taxes Due from Customers [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	1.4	1.6	1.7
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	66.9	65.0	63.6
Energy Efficiency [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	66.9	65.0	63.6
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	131.5	100.3	107.1
Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 82.3	$ 88.8	$ 95.3

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
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

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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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-Publisher FASB
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