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Note 4 - Regulatory Matters (Details Textual) - USD ($) $ in Millions						1 Months Ended		3 Months Ended				6 Months Ended	7 Months Ended	9 Months Ended
	Apr. 21, 2022	Jan. 18, 2022	Jan. 03, 2022	Dec. 23, 2021	Aug. 23, 2021	Oct. 31, 2021	Feb. 28, 2021	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2022	Jun. 30, 2021	Jul. 22, 2022	Jul. 12, 2022	Jul. 11, 2022	Dec. 22, 2021
Utilities Operating Expense, Gas and Petroleum Purchased								$ 203.3			$ 96.9			$ 844.5	$ 897.2
Spire S T L Pipeline L L C [Member]
Asset Impairment Charges, Total										$ 270.0
Revolving Credit Facility [Member] \| Subsequent Event [Member]
Line of Credit Facility, Maximum Borrowing Capacity																$ 1,300.0
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	94.7			55.6			538.8	754.6
Public Utilities, Reduced Revenue Accounts Receivable Cost Of Gas and Regulatory Liabilities									$ 150.0
Equity and Debt Financing, Authority Pending Approval																			$ 800.0
Spire Missouri [Member] \| Revolving Credit Facility [Member] \| Subsequent Event [Member]
Line of Credit Facility, Maximum Borrowing Capacity																575.0
Spire Missouri [Member] \| Infrastructure System Replacement Surcharge [Member]
Regulated Operating Revenue, Pipelines												$ 11.9	$ 11.3
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								30.4				30.4		30.4
Deferred Amount in Accordance With New Capitalization Rates								12.5				12.5		12.5
Public Utilities, Deferred Amount Prudent Costs in Excess of Capitalization Rates								12.9				12.9		12.9
Deferred Amount, Prudent Costs Related To I S R S Settlement								5.0				$ 5.0		5.0
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount														151.9
Regulated Incremental Annual Revenue From Settlement	$ 8.5
Spire Alabama Inc [Member]
Utilities Operating Expense, Gas and Petroleum Purchased								$ 41.0			$ 24.1			$ 138.6	$ 129.0
Spire Alabama Inc [Member] \| Subsequent Event [Member] \| Maximum [Member] \| Spire [Member]
Notes Receivable, Related Parties																	$ 25.0
Spire Alabama Inc [Member] \| Revolving Credit Facility [Member] \| Subsequent Event [Member]
Line of Credit Facility, Maximum Borrowing Capacity																$ 275.0
Spire Alabama Inc [Member] \| Revolving Credit Facility [Member] \| Subsequent Event [Member] \| Spire [Member]
Line of Credit Facility, Maximum Borrowing Capacity																	$ 275.0	$ 200.0
Spire Gulf [Member]
Public Utilities, Approved Return on Equity, Percentage						9.95%
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue			$ 1.0
Spire Mississippi [Member]
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue		$ 0.8			$ 1.1
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 [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)														1 year
PGA Assets Due to Filing Adjustment Factors [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)														3 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