Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 1,020.0	$ 976.4	$ 1,718.7	$ 1,594.4
Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1,014.9	970.2	1,708.2	1,583.7
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1,020.0	976.4	1,718.7	1,594.4
Spire [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1,034.7	990.8	1,748.2	1,623.1
Spire [Member] \| Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(14.7)	(14.4)	(29.5)	(28.7)
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	992.7	977.3	1,677.4	1,569.4
Changes in accrued revenue under alternative revenue programs	22.2	(7.1)	30.8	14.3
Total Gas Utility operating revenues	1,014.9	970.2	1,708.2	1,583.7
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	690.7	679.4	1,165.6	1,090.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	227.4	224.9	375.8	358.6
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	40.2	37.4	77.3	72.0
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	30.7	22.7	50.4	34.2
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.7	12.9	8.3	13.7
Spire [Member] \| Other Operating Segment [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	19.8	20.6	40.0	39.4
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	24.0	(1.3)	33.4	18.5
Operating Revenues	738.3	671.6	1,254.7	1,129.1
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	714.3	672.9	1,221.3	1,110.6
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	516.7	490.0	889.3	811.8
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	152.9	147.7	256.2	241.5
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	12.9	10.8	24.1	20.2
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	26.9	19.5	44.2	29.4
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	4.9	4.9	7.5	7.7
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	(2.1)	(3.6)	(3.2)	(5.4)
Operating Revenues	230.6	252.4	372.3	372.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	232.7	256.0	375.5	378.2
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	146.7	159.5	230.4	231.9
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	60.0	62.5	92.9	91.4
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	24.6	23.8	47.7	46.3
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.8	3.2	6.2	4.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ (2.4)	$ 7.0	$ (1.7)	$ 3.8

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The total amount of operating revenues recognized during the period.

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Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

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The amount of unregulated operating revenues recognized during the period.

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