Note 2 - Revenue - Disaggregated of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Operating Revenues	$ 2,666.3	$ 2,198.5	$ 2,235.5
Consolidation, Eliminations [Member]
Operating Revenues	(52.5)	(51.7)	(48.0)
Gas Utility [Member] | Operating Segments [Member]
Operating Revenues	2,456.9	1,946.1	2,119.3
Gas Marketing [Member] | Operating Segments [Member]
Operating Revenues	179.1	234.9	96.5
Midstream [Member] | Operating Segments [Member]
Operating Revenues	66.1	53.1	52.6
Other Segments [Member] | Consolidation, Eliminations [Member]
Operating Revenues	16.7	16.1	15.1
Spire [Member]
Operating Revenues	2,666.3	2,198.5	2,235.5
Spire [Member] | Operating Segments [Member]
Operating Revenues	2,718.8	2,250.2	2,283.5
Spire [Member] | Consolidation, Eliminations [Member]
Operating Revenues	(52.5)		(48.0)
Spire [Member] | Segment Reconciling Items [Member]
Operating Revenues		(51.7)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	2,422.2	1,935.4	2,117.8
Changes in accrued revenue under alternative revenue programs	34.7	10.7	1.5
Total Gas Utility operating revenues	2,456.9	1,946.1	2,119.3
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	1,648.5	1,416.6	1,234.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	606.0	338.9	586.0
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	121.6	118.0	122.9
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	30.0	42.0	152.7
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	16.1	19.9	22.2
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Unregulated Operating Revenue	179.1	234.9	96.5
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Unregulated Operating Revenue	66.1	53.1	52.6
Spire [Member] | Other Segments [Member] | Operating Segments [Member]
Unregulated Operating Revenue	16.7	16.1	15.1
Spire Missouri [Member]
Changes in accrued revenue under alternative revenue programs	23.9	12.8	3.0
Operating Revenues	1,762.9	1,321.0	1,516.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	1,739.0	1,308.2	1,513.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	1,261.3	1,061.4	882.1
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	411.9	177.2	436.1
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	33.2	33.3	33.5
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	20.1	25.2	145.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	12.5	11.1	16.3
Spire Alabama Inc [Member]
Changes in accrued revenue under alternative revenue programs	6.7	1.7	3.4
Operating Revenues	571.1	509.1	494.0
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	564.4	507.4	490.6
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	322.9	291.6	288.0
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	150.4	119.1	114.9
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	77.6	74.4	78.7
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	9.9	16.8	7.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	$ 3.6	$ 5.5	$ 1.9