Regulatory Matters - Additional Information (Details) - USD ($) $ in Thousands																		1 Months Ended			12 Months Ended
	Oct. 24, 2025	Oct. 23, 2025	Sep. 12, 2025	May 14, 2025	Apr. 17, 2025	Jan. 01, 2025	Dec. 04, 2024	Dec. 01, 2024	Oct. 25, 2024	Oct. 24, 2024	Jan. 01, 2024	Dec. 13, 2023	Dec. 01, 2023	Apr. 30, 2023	Jan. 01, 2023	Apr. 22, 2022	Oct. 01, 2018	Feb. 28, 2025	Sep. 30, 2022	Nov. 30, 2019	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Dec. 01, 2025	Nov. 15, 2024
Public Utilities, General Disclosures [Line Items]
Utilities operating expense, gas and petroleum purchased																					$ 905,500	$ 1,103,300	$ 1,260,800
Spire Missouri [Member]
Public Utilities, General Disclosures [Line Items]
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
Regulatory asset, percentage change of tariff																										30.00%
Utilities operating expense, gas and petroleum purchased																					669,400	886,200	943,400
Public utilities, request rate increase (decrease), amount																		$ 137,400
Public utility rate base amount																					$ 4,379,600
Public utility after-tax total rate of return, infrastructure system replacement surcharge																					7.05%
Public utilities, requested rate increase (decrease), amount					$ 19,000					$ 210,000
Public utilities, approved rate, increase (decrease) annualized revenue				$ 72,600
Spire Missouri [Member] | Infrastructure System Replacement Surcharge [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount																		$ 72,600
Spire Alabama Inc [Member]
Public Utilities, General Disclosures [Line Items]
Utilities operating expense, gas and petroleum purchased																					$ 154,400	189,500	202,700
Public utilities, rate increase decrease index range margin																			1.50%					1.50%
Financing Receivable, after Allowance for Credit Loss																					5,400	5,900
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member]
Public Utilities, General Disclosures [Line Items]
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																					$ 10,000	$ 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		$ 3,600	$ 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, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage																	10.15%		9.50%
Spire Alabama Inc [Member] | Alabama Public Service Commission [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage																	10.65%		9.90%
Spire Alabama Inc [Member] | Subsequent Event [Member] | Alabama Public Service Commission [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, equity limitation as percent of average common equity growth																									8.24%
Annual revenue increase from rates, amount	$ 15,600
Spire Gulf [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage																					9.95%
Public utilities, rate increase decrease index range margin																			1.50%					1.50%
Public utilities approved rate reflected benefit																					$ 200
Annual revenue increase from rates, amount									$ 1,900			$ 2,700
Public utilities, approved rate, increase (decrease) annualized revenue							$ 1,300						$ 2,000	$ 1,800
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, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage																					9.70%
Spire Gulf [Member] | Maximum [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved return on equity, percentage																					10.30%
Spire Gulf [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Annual revenue increase from rates, amount		$ 3,500
Spire [Member] | Maximum [Member] | Related Party [Member]
Public Utilities, General Disclosures [Line Items]
Financing Receivable, after Allowance for Credit Loss																					$ 25,000
Spire [Member] | Alabama Public Service Commission [Member]
Public Utilities, General Disclosures [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity																					$ 75,000
Spire Mississippi [Member]
Public Utilities, General Disclosures [Line Items]
Public utilities, approved rate increase (decrease), amount			$ 800
Public utilities, approved return on equity, percentage																					10.73%
Public utilities, approved rate, increase (decrease) annualized revenue						$ 600					$ 1,000				$ 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]
Public Utilities, General Disclosures [Line Items]
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year)																					20 years
PGA Assets [Member]
Public Utilities, General Disclosures [Line Items]
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year)																					1 year
PGA Assets Not Earning a Return [Member]
Public Utilities, General Disclosures [Line Items]
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year)																					15 years
Related Debt Term and Diaphragm [Member]
Public Utilities, General Disclosures [Line Items]
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year)																					20 years