Note 12 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 5,900	$ 4,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
Spire Missouri [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	10,000	9,900	$ 8,400
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	5,100	3,700
Unrecognized Tax Benefits	22,900	19,300	16,100	$ 13,000
Spire Gulf [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	3,100
Spire Mississippi [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	100
Spire Alabama Inc [Member]
Regulatory Order, Excess Deferred Income Tax Returned to Customers	200
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	300	0
Unrecognized Tax Benefits	$ 300	$ 0	$ 0	$ 0