Note 1 - Summary of Significant Accounting Policies - Transactions with Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Purchases of natural gas from Spire Marketing	$ 13.8	$ 17.9	$ 42.3	$ 36.1
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member] | Related Party [Member]
Transportation services received from Spire STL Pipeline	0.1	0.0	0.1	0.0
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Purchases of natural gas from Spire Marketing	0.0	0.0	1.1	0.0
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member] | Related Party [Member]
Transportation services received from Spire STL Pipeline	$ 7.8	$ 7.8	$ 15.9	$ 15.9