Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2025	Dec. 31, 2024
Disaggregation of Revenue [Line Items]
Operating Revenues		$ 762.2	$ 669.1
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		773.4	680.0
Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		693.3	613.5
Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		41.1	33.0
Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		39.0	33.5
Other Operating Segment [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	[1]	1.0	0.4
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		762.2	669.1
Spire [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		779.0	684.7
Spire [Member] | Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues		(16.8)	(15.6)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		684.7	592.1
Changes in accrued revenue under alternative revenue programs		8.6	21.4
Total Gas Utility operating revenues		693.3	613.5
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		474.9	411.5
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		148.4	133.7
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		37.1	34.6
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		19.7	11.5
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		4.6	0.8
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		41.1	33.0
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		39.0	33.5
Spire [Member] | Other Operating Segment [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue		5.6	4.7
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs		9.4	19.8
Operating Revenues		516.4	457.5
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		507.0	437.7
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		372.6	321.8
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		103.3	93.8
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		11.2	9.4
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		17.3	9.9
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		2.6	2.8
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs		(1.1)	(1.8)
Operating Revenues		141.7	120.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue		142.8	122.2
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue		83.7	72.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue		32.9	28.9
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue		23.1	22.5
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue		2.4	1.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue		$ 0.7	$ (3.2)

[1] All other components of the Company's consolidated information include Spire's subsidiaries engaged in risk management, among other activities, and unallocated corporate items, including certain debt and associated interest costs.