Note 4 - Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Regulatory assets	$ 269.1	$ 355.4	$ 265.7
Regulatory Assets Noncurrent	1,220.7	1,112.4	1,089.9
Total Regulatory Assets	1,489.8	1,467.8	1,355.6
Regulatory liabilities	3.7	3.7	4.1
Regulatory Liability Noncurrent	344.9	418.2	531.8
Total Regulatory Liabilities	348.6	421.9	535.9
Spire Missouri [Member]
Regulatory assets	191.2	288.1	236.9
Regulatory Assets Noncurrent	634.7	547.6	560.5
Total Regulatory Assets	825.9	835.7	797.4
Regulatory liabilities	0.0	0.0	0.2
Regulatory Liability Noncurrent	259.0	331.8	447.2
Total Regulatory Liabilities	259.0	331.8	447.4
Spire Alabama Inc [Member]
Regulatory assets	67.3	56.9	14.6
Regulatory Assets Noncurrent	560.4	538.2	501.7
Total Regulatory Assets	627.7	595.1	516.3
Regulatory liabilities	0.0	0.0	0.8
Regulatory Liability Noncurrent	22.2	23.0	25.3
Total Regulatory Liabilities	22.2	23.0	26.1
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets	238.9	322.2	242.4
Regulatory Assets Noncurrent	107.9	0.0	27.3
Regulatory liabilities	0.0	53.0	155.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Missouri [Member]
Regulatory assets	181.2	275.1	235.2
Regulatory Assets Noncurrent	107.9	0.0	27.3
Regulatory liabilities	0.0	53.0	155.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Alabama Inc [Member]
Regulatory assets	54.5	43.8	4.1
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets	30.2	33.2	23.3
Regulatory Assets Noncurrent	131.7	129.2	94.7
Regulatory liabilities	3.7	3.7	4.1
Regulatory Liability Noncurrent	13.7	14.4	13.8
Other Regulatory Assets (Liabilities) [Member] \| Spire Missouri [Member]
Regulatory assets	10.0	13.0	1.7
Regulatory Assets Noncurrent	116.3	113.1	77.8
Regulatory liabilities	0.0	0.0	0.2
Regulatory Liability Noncurrent	7.0	7.3	8.1
Other Regulatory Assets (Liabilities) [Member] \| Spire Alabama Inc [Member]
Regulatory assets	12.8	13.1	10.5
Regulatory Assets Noncurrent	1.0	1.0	1.2
Regulatory liabilities	0.0	0.0	0.8
Regulatory Liability Noncurrent	3.5	3.6	3.7
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets	277.3	294.5	335.9
Regulatory Assets Noncurrent	156.6	172.6	172.2
Pension and Other Postretirement Plans Costs [Member] \| Spire Missouri [Member]
Regulatory assets	197.4	222.9	241.6
Regulatory Assets Noncurrent	127.4	143.6	142.6
Pension and Other Postretirement Plans Costs [Member] \| Spire Alabama Inc [Member]
Regulatory assets	75.1	66.5	88.7
Regulatory Assets Noncurrent	18.7	19.4	21.6
Deferred Income Tax Charge [Member]
Regulatory Assets Noncurrent	138.6	137.8	135.5
Deferred Income Tax Charge [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	130.1	129.2	126.9
Deferred Income Tax Charge [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	2.1	2.2	2.2
Removal Costs [Member]
Regulatory Assets Noncurrent	506.1	493.7	444.5
Regulatory Liability Noncurrent	33.7	32.9	35.3
Removal Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	23.9	25.2	34.9
Regulatory Liability Noncurrent	0.0	0.0	3.3
Removal Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	482.2	468.5	409.6
Renewable Energy Program [Member]
Regulatory Assets Noncurrent	59.1	57.2	52.0
Renewable Energy Program [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	59.1	57.2	52.0
Estimated Deferred Taxes Due to Customers [Member]
Regulatory Liability Noncurrent	140.9	145.3	155.5
Estimated Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Regulatory Liability Noncurrent	$ 124.6	$ 127.9	$ 138.2

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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