Note 15 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Regulatory assets not earning a return	$ 672.5	$ 686.3
Spire Missouri [Member]
Regulatory assets not earning a return	666.2	679.9
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	133.4	152.9
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	133.4	152.9
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	142.5	135.6
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	136.2	129.2
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	292.4	275.1
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	292.4	275.1
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	104.2	122.7
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 104.2	$ 122.7