Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits, beginning of year	$ 25.3	$ 22.9	$ 19.3
Increases related to tax positions taken in current year	0.2	2.4	3.6
Decreases related to tax positions taken in prior years	24.2	0.0	0.0
Reductions due to lapse of applicable statute of limitations	0.0	0.0	0.0
Unrecognized tax benefits, end of year	1.3	25.3	22.9
Spire Alabama Inc [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits, beginning of year	0.3	0.3	0.0
Increases related to tax positions taken in current year	0.0	0.0	0.3
Decreases related to tax positions taken in prior years	0.0	0.0	0.0
Reductions due to lapse of applicable statute of limitations	0.0	0.0	0.0
Unrecognized tax benefits, end of year	0.3	0.3	0.3
Parent Company [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits, beginning of year	26.3	23.8	19.6
Increases related to tax positions taken in current year	0.6	2.6	4.2
Decreases related to tax positions taken in prior years	(24.2)	0.0	0.0
Reductions due to lapse of applicable statute of limitations	0.0	(0.1)	0.0
Unrecognized tax benefits, end of year	$ 2.7	$ 26.3	$ 23.8