Note 5 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023
Regulatory assets not earning a return	$ 487.2	$ 672.5	$ 607.4
Spire Missouri [Member]
Regulatory assets not earning a return	481.0	666.2	601.1
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	133.8	133.4	127.8
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	133.8	133.4	127.8
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	144.7	142.5	137.2
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	138.5	136.2	130.9
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	81.0	292.4	240.0
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	81.0	292.4	240.0
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	127.7	104.2	102.4
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 127.7	$ 104.2	$ 102.4