Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 669.1	$ 756.6
Consolidation, Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(15.6)	(13.9)
Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	613.5	715.2
Gas Marketing [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	33.0	36.3
Midstream [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	33.5	14.9
Other Operating Segment [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	4.7	4.1
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	19.8	20.8
Operating Revenues	457.5	535.4
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	437.7	514.6
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	321.8	388.8
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	93.8	110.2
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	9.4	9.0
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	9.9	4.1
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	2.8	2.5
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	(1.8)	(1.0)
Operating Revenues	120.4	142.1
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	122.2	143.1
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	72.4	84.4
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	28.9	34.7
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	22.5	20.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	1.6	2.3
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	(3.2)	0.9
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	669.1	756.6
Spire [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	684.7	770.5
Spire [Member] \| Consolidation, Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(15.6)	(13.9)
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	592.1	695.2
Changes in accrued revenue under alternative revenue programs	21.4	20.0
Total Gas Utility operating revenues	613.5	715.2
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	411.5	492.8
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	133.7	157.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	34.6	32.6
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	11.5	6.4
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	0.8	5.5
Spire [Member] \| Gas Marketing [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	33.0	36.3
Spire [Member] \| Midstream [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	33.5	14.9
Spire [Member] \| Other Operating Segment [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	$ 4.7	$ 4.1

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