Note 12 - Income Taxes - Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Total income tax expense	$ 38.8	$ 58.9	$ 68.5
Spire Missouri [Member]
Current	0.0	0.0	0.0
Deferred	10.1	18.2	22.0
Investment tax credits	(0.2)	(0.2)	(0.2)
Current	0.0	0.0	0.0
Deferred	1.7	3.3	3.5
Total income tax expense	11.6	21.3	25.3
Spire Alabama Inc [Member]
Current	0.0	0.0	0.0
Deferred	15.8	18.1	19.8
Investment tax credits	0.0	0.0	0.0
Current	0.0	0.0	0.0
Deferred	4.5	5.1	5.2
Total income tax expense	20.3	23.2	25.0
Parent Company [Member]
Current	0.7	0.5	0.2
Deferred	30.4	47.0	49.5
Investment tax credits	(0.2)	(0.2)	(0.2)
Current	1.2	0.5	1.3
Deferred	6.7	11.1	17.7
Total income tax expense	$ 38.8	$ 58.9	$ 68.5