Segment Information - Additional Information (Details)	12 Months Ended
Sep. 30, 2025 Segment
Segment Reporting Information [Line Items]
Number of reportable segments	3
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	srt:ChiefExecutiveOfficerMember
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description	The CODM and management evaluate the performance of the segments based on the computation of adjusted earnings. Adjusted earnings excludes from reported net income, as applicable, the after-tax impacts of fair value accounting and timing adjustments associated with energy-related transactions, the impacts of acquisition, divestiture and restructuring activities, and the largely non-cash impacts of other non-recurring or unusual items such as certain regulatory, legislative or GAAP standard-setting actions. For each of the Company’s segments, the CODM uses adjusted earnings to allocate resources and determine reinvestment for each segment, predominantly in the annual budget and forecasting process. Adjusted earnings is also used to monitor budget versus actual results to assess performance of the segment and establish management compensation. The CODM does not receive asset information for the individual reportable segments.
Spire Missouri [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1
Spire Alabama Inc [Member]
Segment Reporting Information [Line Items]
Number of reportable segments	1

3

Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description

The CODM and management evaluate the performance of the segments based on the computation of adjusted earnings. Adjusted earnings excludes from reported net income, as applicable, the after-tax impacts of fair value accounting and timing adjustments associated with energy-related transactions, the impacts of acquisition, divestiture and restructuring activities, and the largely non-cash impacts of other non-recurring or unusual items such as certain regulatory, legislative or GAAP standard-setting actions. For each of the Company’s segments, the CODM uses adjusted earnings to allocate resources and determine reinvestment for each segment, predominantly in the annual budget and forecasting process. Adjusted earnings is also used to monitor budget versus actual results to assess performance of the segment and establish management compensation. The CODM does not receive asset information for the individual reportable segments.

Segment Reporting Information [Line Items]

Number of reportable segments

1