Note 2 - Revenue - Disaggregated of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Operating Revenues	$ 2,593.0	$ 2,666.3	$ 2,198.5
Consolidation, Eliminations [Member]
Operating Revenues	(62.4)	(52.5)	(51.7)
Gas Utility [Member] | Operating Segments [Member]
Operating Revenues	2,437.9	2,456.9	1,946.1
Gas Marketing [Member] | Operating Segments [Member]
Operating Revenues	99.2	179.1	234.9
Midstream [Member] | Operating Segments [Member]
Operating Revenues	100.7	66.1	53.1
Other Operating Segment [Member] | Consolidation, Eliminations [Member]
Operating Revenues	17.6	16.7	16.1
Spire [Member]
Operating Revenues	2,593.0	2,666.3	2,198.5
Spire [Member] | Operating Segments [Member]
Operating Revenues	2,655.4	2,718.8	2,250.2
Spire [Member] | Consolidation, Eliminations [Member]
Operating Revenues	(62.4)		(51.7)
Spire [Member] | Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member]
Operating Revenues		(52.5)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	2,386.6	2,422.2	1,935.4
Changes in accrued revenue under alternative revenue programs	51.3	34.7	10.7
Total Gas Utility operating revenues	2,437.9	2,456.9	1,946.1
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	1,617.4	1,648.5	1,416.6
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	581.0	606.0	338.9
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	130.0	121.6	118.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	37.0	30.0	42.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	21.2	16.1	19.9
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Unregulated Operating Revenue	99.2	179.1	234.9
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Unregulated Operating Revenue	100.7	66.1	53.1
Spire [Member] | Other Operating Segment [Member] | Operating Segments [Member]
Unregulated Operating Revenue	17.6	16.7	16.1
Spire Missouri [Member]
Changes in accrued revenue under alternative revenue programs	51.0	23.9	12.8
Operating Revenues	1,737.4	1,762.9	1,321.0
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	1,686.4	1,739.0	1,308.2
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	1,217.7	1,261.3	1,061.4
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	390.9	411.9	177.2
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	34.1	33.2	33.3
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	28.8	20.1	25.2
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	14.9	12.5	11.1
Spire Alabama Inc [Member]
Changes in accrued revenue under alternative revenue programs	0.2	6.7	1.7
Operating Revenues	578.9	571.1	509.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	578.7	564.4	507.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	336.4	322.9	291.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	149.5	150.4	119.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	85.1	77.6	74.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	8.1	9.9	16.8
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	$ (0.4)	$ 3.6	$ 5.5