Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Accounting Policies [Line Items]
Accrued capital expenditures	$ 66.8	$ 80.4	$ 116.5
Spire Missouri [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	45.6	47.2	67.4
Spire Alabama Inc [Member]
Accounting Policies [Line Items]
Accrued capital expenditures	$ 8.5	$ 2.7	$ 14.1

Future cash outflow to pay for purchases of fixed assets that have occurred.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482913/230-10-50-4

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482913/230-10-50-3

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482913/230-10-50-5

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration