Note 5 - Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Regulatory assets	$ 256.4	$ 348.3	$ 269.1
Regulatory Assets Noncurrent	1,266.9	1,249.2	1,220.7
Total Regulatory Assets	1,523.3	1,597.5	1,489.8
Regulatory Liability Current	7.1	7.3	3.7
Regulatory Liability Noncurrent	487.2	472.4	344.9
Total Regulatory Liabilities	494.3	479.7	348.6
Spire Missouri [Member]
Regulatory assets	212.9	293.1	191.2
Regulatory Assets Noncurrent	630.8	617.6	634.7
Total Regulatory Assets	843.7	910.7	825.9
Regulatory Liability Noncurrent	404.5	389.4	259.0
Total Regulatory Liabilities	404.5	389.4	259.0
Spire Alabama Inc [Member]
Regulatory assets	31.0	41.6	67.3
Regulatory Assets Noncurrent	612.6	606.9	560.4
Total Regulatory Assets	643.6	648.5	627.7
Regulatory Liability Noncurrent	20.7	21.2	22.2
Total Regulatory Liabilities	20.7	21.2	22.2
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets	207.0	293.2	238.9
Regulatory Assets Noncurrent	20.3	23.0	107.9
Regulatory Liability Noncurrent	23.7	11.2	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Missouri [Member]
Regulatory assets	194.1	269.4	181.2
Regulatory Assets Noncurrent	20.3	23.0	107.9
Regulatory Liability Noncurrent	23.7	11.2	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Alabama Inc [Member]
Regulatory assets	12.2	21.7	54.5
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets	49.4	55.1	30.2
Regulatory Assets Noncurrent	147.4	131.2	131.7
Regulatory Liability Current	7.1	7.3	3.7
Regulatory Liability Noncurrent	20.5	21.4	13.7
Other Regulatory Assets (Liabilities) [Member] \| Spire Missouri [Member]
Regulatory assets	18.8	23.7	10.0
Regulatory Assets Noncurrent	133.2	116.0	116.3
Regulatory Liability Noncurrent	15.6	16.5	7.0
Other Regulatory Assets (Liabilities) [Member] \| Spire Alabama Inc [Member]
Regulatory assets	18.8	19.9	12.8
Regulatory Assets Noncurrent	1.0	0.9	1.0
Regulatory Liability Noncurrent	3.4	3.3	3.5
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets Noncurrent	255.3	261.0	277.3
Regulatory Liability Noncurrent	188.2	185.2	156.6
Pension and Other Postretirement Plans Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	185.5	189.1	197.4
Regulatory Liability Noncurrent	160.2	156.5	127.4
Pension and Other Postretirement Plans Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	66.0	67.8	75.1
Regulatory Liability Noncurrent	17.3	17.9	18.7
Deferred Income Tax Charge [Member]
Regulatory Assets Noncurrent	145.6	144.5	138.6
Deferred Income Tax Charge [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	137.3	136.2	130.1
Deferred Income Tax Charge [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	1.9	2.0	2.1
Removal Costs [Member]
Regulatory Assets Noncurrent	640.8	633.2	506.1
Regulatory Liability Noncurrent	130.3	126.6	33.7
Removal Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	97.0	97.0	23.9
Regulatory Liability Noncurrent	93.4	90.4	0.0
Removal Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	543.7	536.2	482.2
Renewable Energy Program [Member]
Regulatory Assets Noncurrent	57.5	56.3	59.1
Renewable Energy Program [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	57.5	56.3	59.1
Estimated Deferred Taxes Due to Customers [Member]
Regulatory Liability Noncurrent	124.5	128.0	140.9
Estimated Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Regulatory Liability Noncurrent	$ 111.6	$ 114.8	$ 124.6

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.

+ References

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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