Note 5 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022	Mar. 31, 2022
Regulatory assets not earning a return	$ 607.4	$ 686.3	$ 555.6
Spire Missouri [Member]
Regulatory assets not earning a return	601.1	679.9	549.2
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	127.8	152.9	152.0
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	127.8	152.9	152.0
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	137.2	135.6	135.7
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	130.9	129.2	129.3
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	240.0	275.1	147.7
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	240.0	275.1	147.7
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	102.4	122.7	120.2
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 102.4	$ 122.7	$ 120.2