Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	$ 0.0	$ 3.0	$ 8.3	$ 11.2
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	0.0	0.0	0.0	13.9
Unregulated Operation [Member] \| Spire MoGas Pipeline [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	9.7	9.6	29.6	29.2
Unregulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 0.0	$ 0.0	$ 0.1	$ 0.1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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