Note 5 - Regulatory Matters (Details Textual) - USD ($) $ in Millions								1 Months Ended		3 Months Ended			9 Months Ended
	Jun. 30, 2023	Apr. 30, 2023	Apr. 20, 2023	Jan. 01, 2023	Nov. 18, 2022	Sep. 14, 2022	Dec. 23, 2021	Oct. 31, 2022	Feb. 28, 2021	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022	Apr. 28, 2023	Jan. 25, 2023	Sep. 30, 2022
Utilities Operating Expense, Gas and Petroleum Purchased										$ 169.8	$ 203.3		$ 1,175.5	$ 844.5
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	105.5	94.7		887.9	538.8
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					$ 78.0		72.2
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 24.9
Public Utilities, Deferred Amount Under Order Authorizing Accounting Treatment																	$ 42.8
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue	$ 14.2		$ 7.7
Public Utilities, Disallowance From ACA Balance																$ 0.6
Public Utilities, Disallowance From ACA Balance Filed															$ 27.7
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 19.0
Spire Alabama Inc [Member]
Utilities Operating Expense, Gas and Petroleum Purchased										$ 42.7	$ 41.0		$ 173.9	$ 138.6
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member]
Annual Revenue Increase From Rates, Amount				$ 15.0
Spire Gulf [Member]
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue		$ 1.8						$ 2.5
Public Utilities, Approved Return on Equity, Percentage								9.95%
Spire Mississippi [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount						$ 1.3
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									20 years			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									15 years			15 years