Note 4 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Regulatory assets not earning a return	$ 675.4	$ 686.3	$ 666.6
Spire Missouri [Member]
Regulatory assets not earning a return	669.1	679.9	660.2
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	127.0	152.9	163.4
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	127.0	152.9	163.4
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	136.4	135.6	133.3
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	130.1	129.2	126.9
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	289.1	275.1	262.5
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	289.1	275.1	262.5
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	122.9	122.7	107.4
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 122.9	$ 122.7	$ 107.4