Note 15 - Regulatory Matters (Details Textual) - USD ($) $ in Thousands															1 Months Ended				3 Months Ended	12 Months Ended
	Dec. 01, 2024	Nov. 16, 2024	Oct. 25, 2024	Oct. 24, 2024	Sep. 04, 2024	Jan. 01, 2024	Dec. 13, 2023	Dec. 01, 2023	Apr. 30, 2023	Jan. 01, 2023	Apr. 22, 2022	Feb. 01, 2022	Jan. 01, 2022	Oct. 01, 2018	Sep. 30, 2024	Sep. 30, 2022	Feb. 28, 2021	Nov. 30, 2019	Mar. 31, 2022	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Nov. 15, 2024
Utilities Operating Expense, Gas and Petroleum Purchased																				$ 1,103,300	$ 1,260,800	$ 923,900
Spire Missouri [Member]
Public Utilities Approved Percentage of Excess Natural Gas Supply and Capacity to Customers															75.00%
Public Utilities Approved Percentage of Excess Natural Gas Supply and Capacity To Retain															25.00%
Public Utilities, Value From Capacity Release, Threshold															$ 2,200
Utilities Operating Expense, Gas and Petroleum Purchased																	$ 195,800			886,200	943,400	587,000
Public Utilities, Reduced Revenue Accounts Receivable Cost Of Gas and Regulatory Liabilities																			$ 150,000
Utilities Operating, Due From Other Parties															20,000				$ 42,000	20,000
Public Utilities, Approved Rate Increase (Decrease), Amount																				53,600
Spire Missouri [Member] | Subsequent Event [Member]
Regulatory Asset, Percentage Change of Tariff																							30.00%
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 16,700
Spire Alabama Inc [Member]
Utilities Operating Expense, Gas and Petroleum Purchased																				189,500	202,700	$ 161,500
Public Utilities, Rate Increase Decrease Index Range Margin																1.50%						1.50%
Financing Receivable, after Allowance for Credit Loss															5,900					5,900	6,700
Spire Alabama Inc [Member] | Maximum [Member] | Spire [Member] | Related Party [Member]
Financing Receivable, after Allowance for Credit Loss															25,000					25,000
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member]
Public Utilities Approved Percentage of Excess Natural Gas Supply and Capacity to Customers																		75.00%
Public Utilities Approved Percentage of Excess Natural Gas Supply and Capacity To Retain																		25.00%
Public Utilities, Value From Capacity Release, Threshold											$ 1,600
Public Utilities, Approved Rate Increase (Decrease), Amount																				$ 2,500	$ 4,400
Public Utilities Maximum Return on Equity Percentage of Prior Year Revenues																4.00%						4.00%
Public Utilities, Approved Return on Equity, Percentage														10.40%		9.70%
Public Utilities, Rate Increase Decrease Index Range Margin																0.10%						0.10%
Public Utilities, Equity Limitation as Percent of Total Capitalization																55.50%						55.50%
Public Utilities, Equity Limitation as Percent of Average Common Equity Growth																6.00%						6.00%
Annual Revenue Increase From Rates, Amount	$ 4,000					$ 14,300				$ 15,000
Public Utilities, Requested Rate Increase (Decrease), Amount																						$ 17,200
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Minimum [Member]
Public Utilities, Approved Return on Equity, Percentage														10.15%		9.50%
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Maximum [Member]
Public Utilities, Approved Return on Equity, Percentage														10.65%		9.90%
Spire Alabama Inc [Member] | Subsequent Event [Member] | Alabama Public Service Commission [Member]
Annual Revenue Increase From Rates, Amount				$ 3,600
Spire Gulf [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount																						$ 1,700
Public Utilities, Approved Return on Equity, Percentage																				9.95%
Public Utilities, Rate Increase Decrease Index Range Margin																1.50%						1.50%
Annual Revenue Increase From Rates, Amount							$ 2,700
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue								$ 2,000	$ 1,800	2,500			$ 1,000
Public Utilities, Approved Return on Equity, Pro-Rata, Percentage																				75.00%
Regulated Operating Revenue Variances Amount																				$ 100
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Single Force Majeure Event Deferred Amount																				100
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Multiple Force Majeure Events Deferred Amount																				$ 150
Regulated Operations, Recovery of Incurred Costs, Percentage																				90.00%
Spire Gulf [Member] | Minimum [Member]
Public Utilities, Approved Return on Equity, Percentage																				9.70%
Spire Gulf [Member] | Maximum [Member]
Public Utilities, Approved Return on Equity, Percentage																				10.30%
Spire Gulf [Member] | Subsequent Event [Member] | Forecast [Member]
Annual Revenue Increase From Rates, Amount			$ 1,900
Spire [Member] | Alabama Public Service Commission [Member]
Line of Credit Facility, Maximum Borrowing Capacity															$ 75,000					$ 75,000
Spire Mississippi [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 800
Public Utilities, Approved Return on Equity, Percentage																				10.04%
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue						$ 1,000				$ 800		$ 800
Public Utilities, Approved Return on Equity, Excess of Allowed Return, Percentage																				1.00%
Public Utilities, Approved Return on Equity Shortfall Percentage																				75.00%
Public Utilities, Approved Return On Equity, Excess, Percentage																				50.00%
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 [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)															1 year					1 year
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