Note 6 - Long-term Debt (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Long-Term Debt, Gross	$ 3,771.1	$ 3,732.7
Interest Paid, Capitalized, Investing Activities	17.0	8.6	$ 4.5
Subsidiaries [Member]
Long-Term Debt, Gross	223.1
Spire Missouri [Member]
Long-Term Debt, Gross	1,818.0	1,798.0
Interest Paid, Capitalized, Investing Activities	4.6	3.1	0.6
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized	850.0
Spire Alabama Inc [Member]
Long-Term Debt, Gross	750.0	750.0
Interest Paid, Capitalized, Investing Activities	$ 1.8	$ 2.3	$ 3.2