Note 15 - Regulatory Matters (Details Textual) - USD ($) $ in Thousands															1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 01, 2023	Oct. 04, 2023	Sep. 01, 2023	Apr. 30, 2023	Apr. 20, 2023	Jan. 01, 2023	Nov. 18, 2022	Apr. 22, 2022	Feb. 01, 2022	Jan. 01, 2022	Dec. 29, 2021	Dec. 23, 2021	Oct. 26, 2021	Oct. 01, 2018	Sep. 30, 2022	Feb. 28, 2021	Nov. 30, 2019	Mar. 31, 2022	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Jan. 25, 2023
Customer Share																				75.00%
Utilities Operating Expense, Gas and Petroleum Purchased																				$ 1,260,800	$ 923,900	$ 946,300
Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 47,300
Alabama Public Service Commission [Member] | Enhanced Stability Reserve [Member]
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Deferred Amount																					1,000
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Single Force Majeure Event Deferred Amount																				300
Utilities Operating Expense Maintenance and Operations Self Insurance Cost Multiple Force Majeure Events Deferred Amount																				400
Regulated Operating Revenue Variances Amount																				$ 400
Regulated Operations Enhanced Stability Reserve Underfunded Balances Amortization Term (Year)																				5 years
Regulated Operations, Enhanced Stability Reserve, Unfunded Balances, Amortization, Annual Limitation																				$ 700
Spire Missouri [Member]
Customer Share																				25.00%
Utilities Operating Expense, Gas and Petroleum Purchased																$ 195,800				$ 943,400	587,000	786,800
Public Utilities, Reduced Revenue Accounts Receivable Cost Of Gas and Regulatory Liabilities																		$ 150,000
Utilities Operating, Due From Other Parties																		$ 42,000
Spire Missouri [Member] | Missouri Public Service Commission [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount							$ 78,000					72,200
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 24,900
Public Utilities, Deferred Amount Under Order Authorizing Accounting Treatment															$ 42,800						42,800
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue					$ 7,700
Public Utilities, Disallowance From ACA Balance																							$ 600
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Subsequent Event [Member]
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue		$ 12,400
Spire Missouri [Member] | Missouri Public Service Commission [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount							$ 19,000
Spire Alabama Inc [Member]
Utilities Operating Expense, Gas and Petroleum Purchased																				202,700	$ 161,500	145,300
Public Utilities, Rate Increase Decrease Index Range Margin															1.50%						1.50%
Public Utilities, Approved Point of Test Giveback Under Rate Stabilization and Equalization															$ 2,200						$ 2,200	2,200
Financing Receivable, after Allowance for Credit Loss															$ 7,100					6,700	7,100
Spire Alabama Inc [Member] | Maximum [Member] | Spire [Member] | Related Party [Member]
Financing Receivable, after Allowance for Credit Loss																				25,000
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount										$ 5,300										4,400		8,800
Public Utilities, Requested Rate Increase (Decrease), Amount											$ 5,300										$ 17,200
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						$ 15,000
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%									25.00%
Public Utilities, Value From Capacity Release, Threshold								$ 1,600
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Forecast [Member]
Annual Revenue Increase From Rates, Amount	$ 15,800
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 Gulf [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount																					$ 1,700	2,300
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue				$ 1,800		2,500				$ 1,000
Public Utilities, Approved Return on Equity, Percentage													9.95%								10.70%
Public Utilities, Rate Increase Decrease Index Range Margin															1.50%						1.50%
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 Operating Revenue Variances Amount																				$ 100
Public Utilities, Approved Rate, Net Income Benefit																						$ 1,600
Public Utilities, Approved Return on Equity, Pro-Rata, Percentage																				75.00%
Regulated Operations, Recovery of Incurred Costs, Percentage																				90.00%
Spire Gulf [Member] | Forecast [Member]
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue	$ 2,000
Public Utilities, Approved Return on Equity, Percentage																			9.95%
Spire Gulf [Member] | Minimum [Member]
Public Utilities, Approved Return on Equity, Percentage																					10.45%
Spire Gulf [Member] | Minimum [Member] | Forecast [Member]
Public Utilities, Approved Return on Equity, Percentage																			9.70%
Spire Gulf [Member] | Maximum [Member]
Public Utilities, Approved Return on Equity, Percentage																					10.95%
Spire Gulf [Member] | Maximum [Member] | Forecast [Member]
Public Utilities, Approved Return on Equity, Percentage																			10.30%
Spire [Member] | Alabama Public Service Commission [Member]
Line of Credit Facility, Maximum Borrowing Capacity																				$ 75,000
Spire Mississippi [Member]
Public Utilities, Approved Rate Increase (Decrease), Amount			$ 1,300
Public Utilities, Approved Rate, Increase Decrease Annualized Revenue						$ 800			$ 800
Public Utilities, Approved Return on Equity, Percentage																				10.04%
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
PGA Assets [Member]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)																				2 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