Note 2 - Revenue - Disaggregated of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Operating Revenues	$ 1,123.4	$ 880.9	$ 1,937.4	$ 1,436.3
Operating Segments [Member]
Operating Revenues			24.6
Consolidation, Eliminations [Member]
Operating Revenues	(12.3)	(14.0)		(26.6)
Gas Utility [Member] \| Operating Segments [Member]
Operating Revenues	1,054.5	817.4	1,787.5	1,320.9
Gas Marketing [Member] \| Operating Segments [Member]
Operating Revenues	60.6	59.4	134.7	107.3
Midstream [Member] \| Operating Segments [Member]
Operating Revenues	16.5	13.8	31.7	26.7
Other Segments [Member]
Operating Revenues	4.1	4.3	8.1	8.0
Spire Missouri [Member]
Operating Revenues	791.0	571.2	1,332.2	919.1
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	773.9	574.0	1,313.6	905.3
Changes in accrued revenue under alternative revenue programs	17.1	(2.8)	18.6	13.8
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	570.0	529.1	970.6	776.2
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	185.2	19.9	308.1	88.1
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	9.9	10.1	19.0	19.3
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Revenue	4.5	11.3	9.6	15.8
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	4.3	3.6	6.3	5.9
Spire Alabama Inc [Member]
Operating Revenues	218.3	204.1	370.7	330.7
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	219.8	204.7	372.6	331.4
Changes in accrued revenue under alternative revenue programs	(1.5)	(0.6)	(1.9)	(0.7)
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	135.1	129.9	231.4	210.0
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	61.5	51.6	95.1	77.6
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	20.8	20.7	40.1	40.1
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Revenue	1.7	0.6	4.1	1.0
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	0.7	1.9	1.9	2.7
Spire [Member]
Operating Revenues	1,123.4	880.9	1,937.4	1,436.3
Spire [Member] \| Operating Segments [Member]
Operating Revenues	1,135.7	894.9	1,962.0	1,462.9
Spire [Member] \| Consolidation, Eliminations [Member]
Operating Revenues	(12.3)	(14.0)	(24.6)	(26.6)
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	1,034.2	824.9	1,765.7	1,310.3
Changes in accrued revenue under alternative revenue programs	20.3	(7.5)	21.8	10.6
Total Gas Utility operating revenues	1,054.5	817.4	1,787.5	1,320.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	730.2	687.3	1,248.2	1,029.3
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	260.3	85.6	430.9	189.4
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	33.3	33.5	64.2	64.7
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Incentive [Member]
Revenue	6.1	12.0	13.7	16.8
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	4.3	6.5	8.7	10.1
Spire [Member] \| Gas Marketing [Member] \| Operating Segments [Member]
Unregulated Operating Revenue	60.6	59.4	134.7	107.3
Spire [Member] \| Midstream [Member] \| Operating Segments [Member]
Unregulated Operating Revenue	16.5	13.8	31.7	26.7
Spire [Member] \| Other Segments [Member] \| Operating Segments [Member]
Unregulated Operating Revenue	$ 4.1	$ 4.3	$ 8.1	$ 8.0

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The amount of unregulated operating revenues recognized during the period.

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No definition available.

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