Note 5 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Regulatory assets not earning a return	$ 609.0	$ 672.5	$ 675.4
Spire Missouri [Member]
Regulatory assets not earning a return	602.7	666.2	669.1
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	134.0	133.4	127.0
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	134.0	133.4	127.0
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	143.6	142.5	136.4
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	137.3	136.2	130.1
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	214.4	292.4	289.1
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	214.4	292.4	289.1
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	117.0	104.2	122.9
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 117.0	$ 104.2	$ 122.9