Note 15 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Regulatory assets not earning a return	$ 440.4	$ 672.5
Spire Missouri [Member]
Regulatory assets not earning a return	434.2	666.2
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	129.7	133.4
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	129.7	133.4
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	148.9	142.5
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	142.7	136.2
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	29.3	292.4
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	29.3	292.4
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	132.5	104.2
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 132.5	$ 104.2