Pension Plans and Other Postretirement Benefits - Additional Information (Details)			12 Months Ended
	Dec. 26, 2022 USD ($)	Dec. 25, 2022 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 26, 2021 USD ($)	Dec. 25, 2021 USD ($)	Dec. 23, 2021 USD ($)
Regulatory Asset, Total			$ 1,401,800,000	$ 1,367,200,000
Defined benefit plan, benefit obligation, actuarial losses prior years, percent			0.0040
Defined benefit plan, benefit obligation, actuarial gains from prior years, percent			0.45%
Implementation Period Used To Calculate Return On Plan Assets (Year)			4 years
Gains Or Losses Includible In Cost Amortized To Extent Exceeds Projected Benefit Obligations or Market Related Value of Plan Assets Percentage			10.00%
Regulatory Liability			$ 617,400,000	585,000,000
Defined Contribution Plan, Cost			16,500,000	16,600,000	$ 15,500,000
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			75,800,000	38,500,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			0	2,600,000	0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			2,700,000	200,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			$ 0	$ 6,100,000	$ 0
Spire Missouri [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate			5.55%	5.10%	6.25%
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement			$ 1,500,000	$ 37,300,000
Regulatory Asset, Total			702,100,000	672,000,000
Regulatory Liability			495,200,000	453,500,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits				6,100,000
Defined Contribution Plan, Cost			9,300,000	9,300,000	$ 9,100,000
Spire Missouri [Member] | Pension Costs [Member] | Missouri West [Member]
Regulatory Asset, Total						$ 4,400,000	$ 5,500,000
Spire Missouri [Member] | Pension Costs [Member] | Missouri East [Member]
Regulatory Asset, Total						$ 29,900,000	$ 32,400,000
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri West [Member]
Regulatory Liabilities For Which No Return On Investment During Recovery Period Is Provided	$ 800,000	$ 1,100,000
Remaining Recovery Period of Regulatory Liabilities for which No Return on Investment During Recovery Period is Provided (Year)	8 years	8 years
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri East [Member]
Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided	$ 6,900,000	$ 11,000,000
Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided (Year)		8 years
Additional Remaining Recovery Period of Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided (Year)	8 years
Spire Missouri [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			37,200,000	25,400,000
Anticipated Contributions to Pension Plans Qualified Trusts			22,100,000
Anticipated Contributions To Pension Plans Non Qualified Trusts			600,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			0	2,600,000	0
Spire Missouri [Member] | Pension Plan [Member] | Nonqualified Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			0
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			2,700,000	200,000
Postretirement Benefit Plans, Recovery in Rates Base Allowance								$ 8,600,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			0	6,100,000	0
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri West [Member]
Regulatory Liability	$ 100,000
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri East [Member]
Regulatory Liability	$ 900,000
Spire Alabama Inc [Member]
Regulatory Asset, Total			666,700,000	661,200,000
Regulatory Liability			49,500,000	62,100,000
Defined Contribution Plan, Cost			$ 3,000,000	$ 2,900,000	3,100,000
Spire Alabama Inc [Member] | Pension Plan [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate			5.55%	5.10%
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 30,100,000	$ 12,600,000
Anticipated Contributions to Pension Plans Qualified Trusts			7,000,000
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			$ 0	0	0
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member]
Anticipated Contributions to Pension Plans Qualified Trusts				0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits				$ 0	$ 0
Spire Alabama And Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate			5.50%	5.05%