Note 12 - Income Taxes - Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Total income tax expense	$ 58.7	$ 38.8	$ 58.9
Spire Missouri [Member]
Current	0.0	0.0	0.0
Deferred	13.5	10.1	18.2
Investment tax credits	(0.2)	(0.2)	(0.2)
Current	0.9	0.0	0.0
Deferred	1.5	1.7	3.3
Total income tax expense	15.7	11.6	21.3
Spire Alabama Inc [Member]
Current	0.0	0.0	0.0
Deferred	21.7	15.8	18.1
Investment tax credits	0.0	0.0	0.0
Current	0.0	0.0	0.0
Deferred	5.2	4.5	5.1
Total income tax expense	26.9	20.3	23.2
Parent Company [Member]
Current	0.2	0.7	0.5
Deferred	46.1	30.4	47.0
Investment tax credits	(0.2)	(0.2)	(0.2)
Current	1.5	1.2	0.5
Deferred	11.1	6.7	11.1
Total income tax expense	$ 58.7	$ 38.8	$ 58.9