Note 4 - Regulatory Matters (Details Textual) - USD ($) $ in Millions						1 Months Ended		3 Months Ended
	Jan. 18, 2023	Dec. 27, 2022	Nov. 18, 2022	Sep. 14, 2022	Dec. 23, 2021	Oct. 31, 2022	Feb. 28, 2021	Dec. 31, 2022	Sep. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jan. 25, 2023
Utilities Operating Expense, Gas and Petroleum Purchased								$ 419.2	$ 419.2		$ 249.2	$ 249.2
Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 47.3
Spire Missouri [Member]
Utilities Operating Expense, Gas and Petroleum Purchased							$ 195.8	$ 316.7			$ 156.3
Public Utilities, Reduced Revenue Accounts Receivable Cost Of Gas and Regulatory Liabilities										$ 150.0
Utilities Operating, Due From Other Parties										$ 42.0
Spire Missouri [Member] | Missouri Public Service Commission [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount					72.2
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 24.9
Public Utilities, Deferred Amount Under Order Authorizing Accounting Treatment									42.8
Deferred Amount in Accordance With New Capitalization Rates									19.0
Public Utilities, Deferred Amount Prudent Costs in Excess of Capitalization Rates									18.8
Deferred Amount, Prudent Costs Related To I S R S Settlement									$ 5.0
Public Utilities New Annualized Base Rate Revenue Authorized			$ 78.0
Public Utilities, Incremental Annualized Revenue Per ISRS Filing		$ 8.8
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Subsequent Event [Member]
Public Utilities, Disallowance From ACA Balance													$ 0.6
Spire Gulf [Member]
Public Utilities, Approved Return on Equity, Percentage						9.95%
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue						$ 2.5
Spire Mississippi [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 1.3
Spire Mississippi [Member] | Subsequent Event [Member]
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue	$ 0.8
Future Income Taxes Due From Customers and Pension and Other Postretirement Benefit Costs [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)								20 years
PGA Assets Not Earning A Return [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)								15 years