Note 5 - Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023
Regulatory assets	$ 116.9	$ 348.3	$ 143.0
Regulatory Assets Noncurrent	1,272.7	1,249.2	1,316.7
Total Regulatory Assets	1,389.6	1,597.5	1,459.7
Regulatory Liability Current	25.1	7.3	5.3
Regulatory Liability Noncurrent	557.7	472.4	360.3
Total Regulatory Liabilities	582.8	479.7	365.6
Spire Missouri [Member]
Regulatory assets	89.2	293.1	62.7
Regulatory Assets Noncurrent	630.5	617.6	722.0
Total Regulatory Assets	719.7	910.7	784.7
Regulatory Liability Noncurrent	475.3	389.4	276.9
Total Regulatory Liabilities	475.3	389.4	276.9
Spire Alabama Inc [Member]
Regulatory assets	14.9	41.6	64.8
Regulatory Assets Noncurrent	619.1	606.9	568.3
Total Regulatory Assets	634.0	648.5	633.1
Regulatory Liability Current	18.7	0.0	0.0
Regulatory Liability Noncurrent	20.1	21.2	21.5
Total Regulatory Liabilities	38.8	21.2	21.5
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets	81.4	293.2	115.3
Regulatory Assets Noncurrent	0.0	23.0	181.3
Regulatory Liability Current	19.8	0.0	0.0
Regulatory Liability Noncurrent	90.8	11.2	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Missouri [Member]
Regulatory assets	81.0	269.4	58.7
Regulatory Assets Noncurrent	0.0	23.0	181.3
Regulatory Liability Noncurrent	90.8	11.2	0.0
Unamortized Purchased Gas Adjustments Asset [Member] \| Spire Alabama Inc [Member]
Regulatory assets	0.0	21.7	53.6
Regulatory Liability Current	18.7	0.0	0.0
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets	35.5	55.1	27.7
Regulatory Assets Noncurrent	167.3	131.2	147.2
Regulatory Liability Current	5.3	7.3	5.3
Regulatory Liability Noncurrent	19.9	21.4	28.9
Other Regulatory Assets (Liabilities) [Member] \| Spire Missouri [Member]
Regulatory assets	8.2	23.7	4.0
Regulatory Assets Noncurrent	153.2	116.0	130.7
Regulatory Liability Noncurrent	15.0	16.5	22.9
Other Regulatory Assets (Liabilities) [Member] \| Spire Alabama Inc [Member]
Regulatory assets	14.9	19.9	11.2
Regulatory Assets Noncurrent	0.8	0.9	1.0
Regulatory Liability Noncurrent	3.3	3.3	3.5
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets Noncurrent	251.4	261.0	273.1
Regulatory Liability Noncurrent	192.6	185.2	161.0
Pension and Other Postretirement Plans Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	183.6	189.1	195.5
Regulatory Liability Noncurrent	165.4	156.5	132.7
Pension and Other Postretirement Plans Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	64.2	67.8	73.1
Regulatory Liability Noncurrent	16.8	17.9	18.0
Deferred Income Tax Charge [Member]
Regulatory Assets Noncurrent	146.6	144.5	139.3
Deferred Income Tax Charge [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	138.5	136.2	130.9
Deferred Income Tax Charge [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	1.9	2.0	2.0
Removal Costs [Member]
Regulatory Assets Noncurrent	649.2	633.2	516.4
Regulatory Liability Noncurrent	133.5	126.6	34.6
Removal Costs [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	97.0	97.0	24.2
Regulatory Liability Noncurrent	95.8	90.4	0.0
Removal Costs [Member] \| Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	552.2	536.2	492.2
Renewable Energy Program [Member]
Regulatory Assets Noncurrent	58.2	56.3	59.4
Renewable Energy Program [Member] \| Spire Missouri [Member]
Regulatory Assets Noncurrent	58.2	56.3	59.4
Estimated Deferred Taxes Due to Customers [Member]
Regulatory Liability Noncurrent	120.9	128.0	135.8
Estimated Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Regulatory Liability Noncurrent	$ 108.3	$ 114.8	$ 121.3

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/1943274/2147478742/980-340-50-1

+ Details

Name:	us-gaap_RegulatoryAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

+ Details

Name:	us-gaap_RegulatoryAssetsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

+ Details

Name:	us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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/1943274/2147477571/980-405-25-1

+ Details

Name:	us-gaap_RegulatoryLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_RegulatoryLiabilityCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_RegulatoryLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=sr_SpireMissouriMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sr_SpireAlabamaIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=sr_UnamortizedPurchasedGasAdjustmentsAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_DeferredIncomeTaxChargesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_RemovalCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=us-gaap_RenewableEnergyProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAssetAxis=sr_EstimatedDeferredTaxesDueToCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: