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Note 1 - Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Missouri [Member]
Purchases of natural gas from Spire Marketing	$ 5.0	$ 28.5
Regulated Operation [Member] \| Spire Marketing [Member] \| Spire Alabama Inc [Member]
Purchases of natural gas from Spire Marketing	3.4	1.1
Unregulated Operation [Member] \| Spire S T L Pipeline L L C [Member] \| Spire Missouri [Member]
Transportation services received from Spire STL Pipeline	8.2	8.1
Unregulated Operation [Member] \| Spire Storage Salt Plains LLC [Member] \| Spire Missouri [Member]
Transportation services received from Spire STL Pipeline	$ 0.3	$ 0.0