Pension Plans and Other Postretirement Benefits - Expected Pension Benefit Payments for Succeeding Fiscal Years (Details) $ in Millions	Sep. 30, 2025 USD ($)
Pension Plan [Member]
2026	$ 51.2
2027	46.0
2028	45.3
2029	42.9
2030	41.7
2031-2035	196.2
Pension Plan [Member] \| Spire Missouri [Member]
2026	38.5
2027	33.4
2028	32.6
2029	30.5
2030	29.1
2031-2035	133.5
Pension Plan [Member] \| Spire Alabama Inc [Member]
2026	8.8
2027	8.8
2028	8.5
2029	8.4
2030	8.5
2031-2035	42.8
Other Postretirement Benefits Plan [Member]
2026	13.4
2027	13.0
2028	12.5
2029	12.0
2030	11.6
2031-2035	54.0
Other Postretirement Benefits Plan [Member] \| Spire Missouri [Member]
2026	10.1
2027	9.7
2028	9.3
2029	8.9
2030	8.5
2031-2035	38.3
Other Postretirement Benefits Plan [Member] \| Spire Alabama Inc [Member]
2026	3.2
2027	3.2
2028	3.1
2029	3.0
2030	3.0
2031-2035	$ 15.2

Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

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Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in fourth fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in third fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in second fiscal year following current fiscal year.

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