Income Taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
State and local Tax Expense (Benefit) [Abstract]
Total income tax expense	$ 59.7	$ 58.7	$ 38.8
Spire Missouri [Member]
Federal Tax Expense (Benefit) [Abstract]
Current	0.0	0.0	0.0
Deferred	12.4	13.5	10.1
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local Tax Expense (Benefit) [Abstract]
Current	0.2	0.9	0.0
Deferred	1.4	1.5	1.7
Total income tax expense	13.8	15.7	11.6
Spire Alabama Inc [Member]
Federal Tax Expense (Benefit) [Abstract]
Current	0.0	0.0	0.0
Deferred	22.9	21.7	15.8
Investment tax credits	0.0	0.0	0.0
State and local Tax Expense (Benefit) [Abstract]
Current	0.0	0.0	0.0
Deferred	5.5	5.2	4.5
Total income tax expense	28.4	26.9	20.3
Parent Company [Member]
Federal Tax Expense (Benefit) [Abstract]
Current	1.7	0.2	0.7
Deferred	46.3	46.1	30.4
Investment tax credits	(0.2)	(0.2)	(0.2)
State and local Tax Expense (Benefit) [Abstract]
Current	0.7	1.5	1.2
Deferred	11.2	11.1	6.7
Total income tax expense	$ 59.7	$ 58.7	$ 38.8