Note 2 - Revenue - Disaggregated of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Operating Revenues	$ 1,128.5	$ 1,123.4	$ 1,885.1	$ 1,937.4
Consolidation, Eliminations [Member]
Operating Revenues	(15.8)	(12.3)	(29.7)	(24.6)
Gas Utility [Member] | Operating Segments [Member]
Operating Revenues	1,072.7	1,054.5	1,787.9	1,787.5
Gas Marketing [Member] | Operating Segments [Member]
Operating Revenues	46.0	60.6	82.3	134.7
Midstream [Member] | Operating Segments [Member]
Operating Revenues	21.5	16.5	36.4	31.7
Other Operating Segment [Member] | Consolidation, Eliminations [Member]
Operating Revenues	4.1	4.1	8.2	8.1
Spire Missouri [Member]
Operating Revenues	766.5	791.0	1,301.9	1,332.2
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	746.0	773.9	1,260.6	1,313.6
Changes in accrued revenue under alternative revenue programs	20.5	17.1	41.3	18.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	532.9	570.0	921.7	970.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	185.7	185.2	295.9	308.1
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	10.1	9.9	19.1	19.0
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	12.3	4.5	16.4	9.6
Spire Missouri [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	5.0	4.3	7.5	6.3
Spire Alabama Inc [Member]
Operating Revenues	259.5	218.3	401.6	370.7
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	263.5	219.8	406.6	372.6
Changes in accrued revenue under alternative revenue programs	(4.0)	(1.5)	(5.0)	(1.9)
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	169.3	135.1	253.7	231.4
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	68.8	61.5	103.5	95.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	24.0	20.8	44.8	40.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	0.7	1.7	3.0	4.1
Spire Alabama Inc [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	0.7	0.7	1.6	1.9
Spire [Member]
Operating Revenues	1,128.5	1,123.4	1,885.1	1,937.4
Spire [Member] | Operating Segments [Member]
Operating Revenues	1,144.3	1,135.7	1,914.8	1,962.0
Spire [Member] | Consolidation, Eliminations [Member]
Operating Revenues	(15.8)
Spire [Member] | Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment [Member]
Operating Revenues		(12.3)	(29.7)	(24.6)
Spire [Member] | Gas Utility [Member] | Operating Segments [Member]
Revenue	1,055.4	1,034.2	1,750.6	1,765.7
Changes in accrued revenue under alternative revenue programs	17.3	20.3	37.3	21.8
Total Gas Utility operating revenues	1,072.7	1,054.5	1,787.9	1,787.5
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Residential [Member]
Revenue	729.5	730.2	1,222.3	1,248.2
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Commercial and Industrial [Member]
Revenue	267.9	260.3	425.8	430.9
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Transportation [Member]
Revenue	36.8	33.3	69.4	64.2
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Off-system and Other Incentive [Member]
Revenue	13.0	6.1	19.4	13.7
Spire [Member] | Gas Utility [Member] | Operating Segments [Member] | Other Customer Revenue [Member]
Revenue	8.2	4.3	13.7	8.7
Spire [Member] | Gas Marketing [Member] | Operating Segments [Member]
Unregulated Operating Revenue	46.0	60.6	82.3	134.7
Spire [Member] | Midstream [Member] | Operating Segments [Member]
Unregulated Operating Revenue	21.5	16.5	36.4	31.7
Spire [Member] | Other Operating Segment [Member] | Operating Segments [Member]
Unregulated Operating Revenue	$ 4.1	$ 4.1	$ 8.2	$ 8.1