Note 1 - Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Transportation services received from Spire STL Pipeline	$ 0.3	$ 0.0	$ 0.4	$ 0.0
Affiliated Entity [Member] | Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Purchases of natural gas from Spire Marketing	6.5	26.2	48.8	62.3
Affiliated Entity [Member] | Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Purchases of natural gas from Spire Marketing	1.7	3.2	2.8	3.2
Affiliated Entity [Member] | Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Transportation services received from Spire STL Pipeline	$ 8.0	$ 8.0	$ 23.9	$ 23.9