Note 5 - Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Regulatory assets	$ 93.3	$ 355.4	$ 152.0
Regulatory Assets Noncurrent	1,421.2	1,112.4	1,205.6
Total Regulatory Assets	1,514.5	1,467.8	1,357.6
Regulatory Liability Current	5.5	3.7	3.2
Regulatory Liability Noncurrent	428.1	418.2	389.0
Total Regulatory Liabilities	433.6	421.9	392.2
Spire Missouri [Member]
Regulatory assets	35.0	288.1	89.9
Regulatory Assets Noncurrent	819.1	547.6	664.6
Total Regulatory Assets	854.1	835.7	754.5
Regulatory Liability Noncurrent	346.1	331.8	300.8
Total Regulatory Liabilities	346.1	331.8	300.8
Spire Alabama Inc [Member]
Regulatory assets	43.9	56.9	52.9
Regulatory Assets Noncurrent	576.7	538.2	514.1
Total Regulatory Assets	620.6	595.1	567.0
Regulatory Liability Noncurrent	20.8	23.0	24.0
Total Regulatory Liabilities	20.8	23.0	24.0
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets	66.2	322.2	123.4
Regulatory Assets Noncurrent	206.3	0.0	97.4
Regulatory Liability Current	0.0	53.0	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Missouri [Member]
Regulatory assets	32.9	275.1	89.7
Regulatory Assets Noncurrent	206.3	0.0	97.4
Regulatory Liability Current	0.0	53.0	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Alabama Inc [Member]
Regulatory assets	30.9	43.8	31.6
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets	27.1	33.2	28.6
Regulatory Assets Noncurrent	152.3	129.2	124.3
Regulatory Liability Current	5.5	3.7	3.2
Regulatory Liability Noncurrent	23.3	14.4	14.6
Other Regulatory Assets (Liabilities) [Member] \| Spire Missouri [Member]
Regulatory assets	2.1	13.0	0.2
Regulatory Assets Noncurrent	136.5	113.1	107.9
Regulatory Liability Current	17.8	7.3	7.7
Other Regulatory Assets (Liabilities) [Member] \| Spire Alabama Inc [Member]
Regulatory assets	13.0	13.1	21.3
Regulatory Assets Noncurrent	1.0	1.0	1.1
Regulatory Liability Current	3.5	3.6	3.6
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets	268.8	294.5	317.5
Regulatory Assets Noncurrent	165.3	172.6	185.0
Pension and Other Postretirement Plans Costs [Member] \| Spire Missouri [Member]
Regulatory assets	193.4	222.9	238.5
Regulatory Assets Noncurrent	137.9	143.6	154.5
Pension and Other Postretirement Plans Costs [Member] \| Spire Alabama Inc [Member]
Regulatory assets	71.1	66.5	73.8
Regulatory Assets Noncurrent	17.3	19.4	20.4
Deferred Income Tax Charge [Member]
Regulatory Assets Noncurrent	140.1	137.8	140.4
Deferred Income Tax Charge [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	131.8	129.2	131.8
Deferred Income Tax Charge [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	2.0	2.2	2.2
Removal Costs [Member]
Regulatory Assets Noncurrent	597.9	493.7	471.9
Regulatory Liability Noncurrent	107.7	32.9	40.7
Removal Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	95.3	25.2	34.9
Regulatory Liability Noncurrent	72.3	0.0	7.3
Removal Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	502.6	468.5	437.0
Renewable Energy Program [Member]
Regulatory Assets Noncurrent	55.8	57.2	54.1
Renewable Energy Program [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	55.8	57.2	54.1
Estimated Deferred Taxes Due to Customers [Member]
Regulatory Liability Noncurrent	131.8	145.3	148.7
Estimated Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Regulatory Liability Noncurrent	$ 118.1	$ 127.9	$ 131.3

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481878/980-340-50-1

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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//1943274/2147481854/980-405-25-1

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

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