Note 12 - Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Spire Missouri [Member]
Unrecognized tax benefits, beginning of year	$ 19,300	$ 16,100	$ 13,000
Increases related to tax positions taken in current year	3,600	3,200	3,100
Reductions due to lapse of applicable statute of limitations	0	0	0
Unrecognized tax benefits, end of year	22,900	19,300	16,100
Spire Alabama Inc [Member]
Unrecognized tax benefits, beginning of year	0	0	0
Increases related to tax positions taken in current year	300	0	0
Reductions due to lapse of applicable statute of limitations	0	0	0
Unrecognized tax benefits, end of year	300	0	0
Parent Company [Member]
Unrecognized tax benefits, beginning of year	19,600	16,400	13,200
Increases related to tax positions taken in current year	4,200	3,300	3,200
Reductions due to lapse of applicable statute of limitations	0	(100)	0
Unrecognized tax benefits, end of year	$ 23,800	$ 19,600	$ 16,400