Note 2 - Revenue - Disaggregated of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Operating Revenues	$ 448.0	$ 327.8	$ 1,884.3	$ 1,945.3
Consolidation, Eliminations [Member]
Operating Revenues	(12.2)		(38.8)	(36.1)
Segment Reconciling Items [Member]
Operating Revenues		(11.8)
Gas Utility [Member] \| Operating Segments [Member]
Operating Revenues	377.4	306.7	1,698.3	1,858.0
Gas Marketing [Member] \| Operating Segments [Member]
Operating Revenues	64.1	15.1	171.4	73.3
Other Segments [Member] \| Operating Segments [Member]
Operating Revenues	18.7	17.8	53.4	50.1
Spire Missouri [Member]
Operating Revenues	236.7	192.1	1,155.8	1,338.5
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	237.8	193.3	1,143.1	1,335.8
Changes in accrued revenue under alternative revenue programs	(1.1)	(1.2)	12.7	2.7
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	171.1	144.4	947.3	772.8
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	51.9	35.0	140.0	408.9
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	7.1	7.4	26.4	26.3
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Inventive [Member]
Revenue	4.5	4.5	20.3	140.4
Spire Missouri [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	3.2	2.0	9.1	(12.6)
Spire Alabama Inc [Member]
Operating Revenues	115.7	93.3	446.4	429.5
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	104.7	89.6	436.1	429.2
Changes in accrued revenue under alternative revenue programs	11.0	3.7	10.3	0.3
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	56.2	45.6	266.2	259.6
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	24.0	21.3	101.6	98.8
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	17.4	18.7	57.5	60.9
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Inventive [Member]
Revenue	5.6	2.8	6.6	6.7
Spire Alabama Inc [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	1.5	1.2	4.2	3.2
Spire [Member]
Operating Revenues	448.0	327.8	1,884.3	1,945.3
Spire [Member] \| Operating Segments [Member]
Operating Revenues	460.2	339.6	1,923.1	1,981.4
Spire [Member] \| Consolidation, Eliminations [Member]
Operating Revenues	(12.2)		(38.8)	(36.1)
Spire [Member] \| Segment Reconciling Items [Member]
Operating Revenues		(11.8)
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member]
Revenue	368.3	303.8	1,678.6	1,860.1
Changes in accrued revenue under alternative revenue programs	9.1	2.9	19.7	(2.1)
Total Gas Utility operating revenues	377.4	306.7	1,698.3	1,858.0
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Residential [Member]
Revenue	239.5	200.4	1,268.8	1,087.9
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Commercial and Industrial [Member]
Revenue	86.2	63.6	275.6	535.8
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Transportation [Member]
Revenue	26.9	28.7	91.6	95.1
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Off-system and Other Inventive [Member]
Revenue	10.1	7.2	26.9	147.1
Spire [Member] \| Gas Utility [Member] \| Operating Segments [Member] \| Other Customer Revenue [Member]
Revenue	5.6	3.9	15.7	(5.8)
Spire [Member] \| Gas Marketing [Member] \| Operating Segments [Member]
Unregulated Operating Revenue	64.1	15.1	171.4	73.3
Spire [Member] \| Other Segments [Member] \| Operating Segments [Member]
Unregulated Operating Revenue	$ 18.7	$ 17.8	$ 53.4	$ 50.1

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The amount of unregulated operating revenues recognized during the period.

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