Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 420.2	$ 352.5	$ 2,138.9	$ 1,946.9
Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	416.1	347.5	2,124.3	1,931.2
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	420.2	352.5	2,138.9	1,946.9
Spire [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	439.5	371.8	2,187.7	1,994.9
Spire [Member] \| Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(19.3)	(19.3)	(48.8)	(48.0)
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	401.1	345.0	2,078.5	1,914.4
Changes in accrued revenue under alternative revenue programs	15.0	2.5	45.8	16.8
Total Gas Utility operating revenues	416.1	347.5	2,124.3	1,931.2
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	233.2	212.0	1,398.8	1,302.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	104.4	79.5	480.2	438.1
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	36.3	30.9	113.6	102.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	19.7	17.0	70.1	51.2
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.5	5.6	15.8	19.3
Spire [Member] \| Other Operating Segment [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	23.4	24.3	63.4	63.7
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	10.0	4.0	43.4	22.5
Operating Revenues	248.8	230.1	1,503.5	1,359.2
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	238.8	226.1	1,460.1	1,336.7
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	159.0	155.8	1,048.3	967.6
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	51.2	46.9	307.4	288.4
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	9.4	7.9	33.5	28.1
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	15.6	12.0	59.8	41.4
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.6	3.5	11.1	11.2
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	2.8	(0.8)	(0.4)	(6.2)
Operating Revenues	99.0	98.8	471.3	471.6
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	96.2	99.6	471.7	477.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	45.2	48.3	275.6	280.2
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	25.2	24.7	118.1	116.1
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	20.6	20.5	68.3	66.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	4.1	5.0	10.3	9.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	$ 1.1	$ 1.1	$ (0.6)	$ 4.9

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The amount of unregulated operating revenues recognized during the period.

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