Note 4 - Regulatory Matters - Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021	Jun. 30, 2021
Regulatory assets not earning a return	$ 622.1	$ 625.2	$ 452.9
Spire Missouri [Member]
Regulatory assets not earning a return	615.7	618.7	446.4
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets not earning a return	165.8	165.7	194.4
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	165.8	165.7	194.4
Deferred Income Tax Charge [Member]
Regulatory assets not earning a return	138.2	130.7	128.0
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	131.8	124.2	121.5
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets not earning a return	187.2	242.8	0.0
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	187.2	242.8	0.0
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets not earning a return	130.9	86.0	130.5
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets not earning a return	$ 130.9	$ 86.0	$ 130.5