Note 5 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Regulatory assets not earning a return	$ 606.9	$ 686.3	$ 622.1
Spire Missouri [Member]
Regulatory assets not earning a return	600.7	679.9	615.7
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	127.5	152.9	165.8
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	127.5	152.9	165.8
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	138.0	135.6	138.2
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	131.8	129.2	131.8
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	239.2	275.1	187.2
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	239.2	275.1	187.2
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	102.2	122.7	130.9
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 102.2	$ 122.7	$ 130.9