Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 78.6	$ 78.3	$ 35.0
Regulatory Assets, Noncurrent	1,395.0	1,323.5	1,293.4
Total Regulatory Assets	1,473.6	1,401.8	1,328.4
Regulatory Liabilities, Current	11.3	39.4	48.6
Regulatory Liabilities, Noncurrent	777.6	578.0	582.0
Total Regulatory Liabilities	788.9	617.4	630.6
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	7.7	48.9	8.7
Regulatory Assets, Noncurrent	669.2	653.2	613.5
Total Regulatory Assets	676.9	702.1	622.2
Regulatory Liabilities, Current	3.5	13.9	14.0
Regulatory Liabilities, Noncurrent	509.5	481.3	491.1
Total Regulatory Liabilities	513.0	495.2	505.1
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	25.8	16.1	13.2
Regulatory Assets, Noncurrent	667.6	650.6	659.6
Total Regulatory Assets	693.4	666.7	672.8
Regulatory Liabilities, Current	1.9	20.3	29.3
Regulatory Liabilities, Noncurrent	28.0	29.2	25.8
Total Regulatory Liabilities	29.9	49.5	55.1
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	3.2	0.0	0.0
Regulatory Assets, Noncurrent	203.4	217.8	236.4
Regulatory Liabilities, Noncurrent	304.7	300.1	261.1
Pension and Postretirement Benefit Costs [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	144.1	157.0	172.5
Regulatory Liabilities, Noncurrent	267.3	261.2	227.2
Pension and Postretirement Benefit Costs [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	56.1	59.1	61.8
Regulatory Liabilities, Noncurrent	25.4	26.5	23.0
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.7	16.5	1.2
Regulatory Assets, Noncurrent	2.1	9.0	0.0
Regulatory Liabilities, Current	3.5	34.9	44.0
Regulatory Liabilities, Noncurrent	37.0	6.5	44.8
Unamortized Purchased Gas Adjustments [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	1.8	14.9	0.0
Regulatory Assets, Noncurrent	2.1	9.0	0.0
Regulatory Liabilities, Current	3.5	13.9	14.0
Regulatory Liabilities, Noncurrent	37.0	6.5	44.8
Unamortized Purchased Gas Adjustments [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	10.7	0.0	0.0
Regulatory Liabilities, Current	0.0	20.2	29.3
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	59.7	61.8	33.8
Regulatory Assets, Noncurrent	254.8	189.2	151.1
Regulatory Liabilities, Current	7.8	4.5	4.6
Regulatory Liabilities, Noncurrent	31.9	32.9	34.1
Other [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	5.9	34.0	8.7
Regulatory Assets, Noncurrent	209.2	176.8	138.7
Regulatory Liabilities, Noncurrent	27.3	28.8	30.2
Other [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.1	16.1	13.2
Regulatory Assets, Noncurrent	0.4	0.3	0.5
Regulatory Liabilities, Current	1.9	0.1	0.0
Regulatory Liabilities, Noncurrent	2.6	2.7	2.8
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	711.8	688.3	687.2
Regulatory Liabilities, Noncurrent	276.0	138.2	138.3
Cost of Removal [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	98.3	98.8	91.4
Regulatory Liabilities, Noncurrent	98.9	96.0	96.9
Cost of Removal [Member] \| Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	609.7	589.6	595.7
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	155.7	154.2	155.0
Future Income Taxes Due from Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	148.3	146.6	147.2
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	1.6
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	67.2	65.0	63.7
Energy Efficiency [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	67.2	65.0	63.7
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	128.0	100.3	103.7
Deferred Taxes Due to Customers [Member] \| Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 79.0	$ 88.8	$ 92.0

Schedule of regulatory assets and liabilities.

No definition available.

Name:	sr_ScheduleOfRegulatoryAssetsAndLiabilitiesLineItems
Namespace Prefix:	sr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

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://www.xbrl.org/2009/role/commonPracticeRef
-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=us-gaap_PensionAndOtherPostretirementPlansCostsMember
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_RemovalCostsMember
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_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: