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Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	$ 32.0	$ 32.5	$ 32.0
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Transaction amount	0.0	0.0	0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	0.0	0.7	0.7
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Transaction amount	0.0	0.0	0.0
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Transaction amount	7.0	4.8	0.0
Unregulated Operation [Member] | Spire MoGas Pipeline [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Transaction amount	0.0	0.0	0.0
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	23.8	13.9	57.4
Transaction amount	0.1	1.7	0.4
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Related Party Transaction [Line Items]
Purchases of natural gas from Spire Marketing	13.9	6.7	4.7
Transaction amount	$ 0.0	$ 0.0	$ 0.0

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-Topic 850
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