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Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Operating Revenues		$ 2,476.4	$ 2,593.0	$ 2,666.3
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		2,520.3	2,637.8	2,702.1
Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		2,207.6	2,437.9	2,456.9
Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		157.2	99.2	179.1
Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		155.5	100.7	66.1
Other [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	[1]	2.5	0.7	0.7
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		2,476.4	2,593.0	2,666.3
Spire [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		2,540.8	2,655.4	2,718.8
Spire [Member] | Intercompany Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		(64.4)	(62.4)	(52.5)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		2,187.2	2,386.6	2,422.2
Changes in accrued revenue under alternative revenue programs		20.3	51.3	34.7
Total Gas Utility operating revenues		2,207.5	2,437.9	2,456.9
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		1,462.6	1,617.4	1,648.5
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		501.3	581.0	606.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		133.6	130.0	121.6
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		65.3	37.0	30.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		24.4	21.2	16.1
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		157.2	99.2	179.1
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		155.5	100.7	66.1
Spire [Member] | Other [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		20.6	17.6	16.7
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs		24.2	51.0	23.9
Operating Revenues		1,544.1	1,737.4	1,762.9
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		1,519.9	1,686.4	1,739.0
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		1,092.1	1,217.7	1,261.3
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		326.0	390.9	411.9
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		35.9	34.1	33.2
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		51.9	28.8	20.1
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		14.0	14.9	12.5
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs		(3.8)	0.2	6.7
Operating Revenues		545.2	578.9	571.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		549.0	578.7	564.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		307.5	336.4	322.9
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		134.5	149.5	150.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		87.3	85.1	77.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		13.4	8.1	9.9
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 6.3	$ (0.4)	$ 3.6
[1] All other components of the Company's consolidated information include Spire's subsidiaries engaged in the operation of a propane pipeline and risk management, among other activities, and unallocated corporate items, including certain debt and associated interest costs.

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