Pension Plans and Other Postretirement Benefits - Additional Information (Details)	6 Months Ended
Mar. 31, 2026 USD ($)
Spire Missouri [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, contributions by employer	$ 8,800,000
Spire Missouri [Member] \| Pension Plan [Member] \| Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated contributions to pension plans qualified trusts	13,300,000
Spire Missouri [Member] \| Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, contributions by employer	0
Anticipated contributions to pension plans qualified trusts	0
Spire Alabama Inc [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, contributions by employer	2,800,000
Anticipated contributions to pension plans qualified trusts	4,200,000
Spire Alabama Inc [Member] \| Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, contributions by employer	0
Anticipated contributions to pension plans qualified trusts	$ 0

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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