Note 12 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 6,200	$ 5,900
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
Spire Missouri [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	9,900	10,000	$ 9,900
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	5,300	5,100
Unrecognized Tax Benefits	25,300	22,900	19,300	$ 16,100
Spire Gulf [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	600	3,100
Spire Mississippi [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	100	100
Spire Alabama Inc [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	200	200
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	300
Unrecognized Tax Benefits	$ 300	$ 300	$ 0	$ 0