Note 5 - Regulatory Matters (Details Textual) - USD ($)						1 Months Ended		3 Months Ended		6 Months Ended
	Apr. 23, 2023	Jan. 01, 2023	Nov. 18, 2022	Sep. 14, 2022	Dec. 23, 2021	Oct. 31, 2022	Feb. 28, 2021	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Apr. 28, 2023	Jan. 25, 2023	Sep. 30, 2022
Utilities Operating Expense, Gas and Petroleum Purchased								$ 586,500,000	$ 392,000,000.0	$ 1,005,700,000	$ 641,200,000
Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 47,300,000
Spire Missouri [Member]
Utilities Operating Expense, Gas and Petroleum Purchased							$ 195,800,000	465,700,000	287,800,000	782,400,000	444,100,000
Public Utilities, Reduced Revenue Accounts Receivable Cost Of Gas and Regulatory Liabilities									150,000,000
Utilities Operating, Due From Other Parties									42,000,000		42,000,000
Spire Missouri [Member] | Missouri Public Service Commission [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 78,000,000.0		72,200,000
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 24,900,000
Public Utilities, Deferred Amount Under Order Authorizing Accounting Treatment														$ 42.8
Public Utilities, Disallowance From ACA Balance													$ 600,000
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Subsequent Event [Member]
Public Utilities, Disallowance From ACA Balance Filed												$ 27,700,000
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Maximum [Member] | Subsequent Event [Member]
Regulated Operating Revenue, Pipelines	$ 7.7
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 19,000,000.0
Spire Alabama Inc [Member]
Utilities Operating Expense, Gas and Petroleum Purchased								$ 61,800,000	$ 52,100,000	$ 131,200,000	$ 97,600,000
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member]
Annual Revenue Increase From Rates, Amount		$ 15,000,000.0
Spire Gulf [Member]
Public Utilities, Approved Return on Equity, Percentage						9.95%
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue						$ 2,500,000
Spire Mississippi [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 1,300,000
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue		$ 800,000
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
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