Revenue - Schedule of Revenue Disaggregated by Source and Customer Type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 1,051.3	$ 1,128.5	$ 1,720.4	$ 1,885.1
Consolidation, Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(15.8)	(15.8)	(31.4)	(29.7)
Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	970.2	1,072.7	1,583.7	1,787.9
Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	53.6	46.0	86.6	82.3
Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	38.4	21.5	71.9	36.4
Other Operating Segment [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	4.9	4.1	9.6	8.2
Spire Missouri [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	(1.3)	20.5	18.5	41.3
Operating Revenues	671.6	766.5	1,129.1	1,301.9
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	672.9	746.0	1,110.6	1,260.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	490.0	532.9	811.8	921.7
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	147.7	185.7	241.5	295.9
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	10.8	10.1	20.2	19.1
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	19.5	12.3	29.4	16.4
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	4.9	5.0	7.7	7.5
Spire Alabama Inc [Member]
Disaggregation of Revenue [Line Items]
Changes in accrued revenue under alternative revenue programs	(3.6)	(4.0)	(5.4)	(5.0)
Operating Revenues	252.4	259.5	372.8	401.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	256.0	263.5	378.2	406.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	159.5	169.3	231.9	253.7
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	62.5	68.8	91.4	103.5
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	23.8	24.0	46.3	44.8
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	3.2	0.7	4.8	3.0
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	7.0	0.7	3.8	1.6
Spire [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1,051.3	1,128.5	1,720.4	1,885.1
Spire [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1,067.1	1,144.3	1,751.8	1,914.8
Spire [Member] | Consolidation, Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(15.8)	(15.8)	(31.4)	(29.7)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue	977.3	1,055.4	1,569.4	1,750.6
Changes in accrued revenue under alternative revenue programs	(7.1)	17.3	14.3	37.3
Total Gas Utility operating revenues	970.2	1,072.7	1,583.7	1,787.9
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue	679.4	729.5	1,090.9	1,222.3
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue	224.9	267.9	358.6	425.8
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Disaggregation of Revenue [Line Items]
Revenue	37.4	36.8	72.0	69.4
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Disaggregation of Revenue [Line Items]
Revenue	22.7	13.0	34.2	19.4
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Disaggregation of Revenue [Line Items]
Revenue	12.9	8.2	13.7	13.7
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	53.6	46.0	86.6	82.3
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	38.4	21.5	71.9	36.4
Spire [Member] | Other Operating Segment [Member] | Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Unregulated Operating Revenue	$ 4.9	$ 4.1	$ 9.6	$ 8.2