Note 12 - Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Spire Missouri [Member]
Unrecognized tax benefits, beginning of year	$ 22,900	$ 19,300	$ 16,100
Increases related to tax positions taken in current year	2,400	3,600	3,200
Reductions due to lapse of applicable statute of limitations	0	0	0
Unrecognized tax benefits, end of year	25,300	22,900	19,300
Spire Alabama Inc [Member]
Unrecognized tax benefits, beginning of year	300	0	0
Increases related to tax positions taken in current year	0	300	0
Reductions due to lapse of applicable statute of limitations	0	0	0
Unrecognized tax benefits, end of year	300	300	0
Parent Company [Member]
Unrecognized tax benefits, beginning of year	23,800	19,600	16,400
Increases related to tax positions taken in current year	2,600	4,200	3,300
Reductions due to lapse of applicable statute of limitations	(100)	0	(100)
Unrecognized tax benefits, end of year	$ 26,300	$ 23,800	$ 19,600