Note 1 - Summary of Significant Accounting Policies - Transactions With Affiliates (Details) - Affiliated Entity [Member] - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Regulated Operation [Member] | Spire Marketing [Member] | Spire Missouri [Member]
Natural gas purchases from Spire Marketing	$ 57.4	$ 86.3	$ 92.5
Transaction amount	0.4	0.0	1.1
Regulated Operation [Member] | Spire Marketing [Member] | Spire Alabama Inc [Member]
Natural gas purchases from Spire Marketing	4.7	3.2	10.4
Transaction amount	0.0	0.5	0.1
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Missouri [Member]
Transaction amount	32.0	32.0	32.0
Unregulated Operation [Member] | Spire S T L Pipeline L L C [Member] | Spire Alabama Inc [Member]
Transaction amount	0.0	0.0	0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Missouri [Member]
Transaction amount	0.7	0.0	0.0
Unregulated Operation [Member] | Spire Storage Salt Plains LLC [Member] | Spire Alabama Inc [Member]
Transaction amount	0.0	0.0	0.0
Unregulated Operation [Member] | Spire N G L Inc [Member] | Spire Missouri [Member]
Transaction amount	0.0	0.0	0.5
Unregulated Operation [Member] | Spire N G L Inc [Member] | Spire Alabama Inc [Member]
Transaction amount	$ 0.0	$ 0.0	$ 0.0