Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 89.6	$ 115.4	$ 256.4
Regulatory Assets, Noncurrent	1,288.4	1,251.8	1,266.9
Total Regulatory Assets	1,378.0	1,367.2	1,523.3
Regulatory Liabilities, Current	51.0	49.5	7.1
Regulatory Liabilities, Noncurrent	577.2	535.5	487.2
Total Regulatory Liabilities	628.2	585.0	494.3
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	56.7	84.0	212.9
Regulatory Assets, Noncurrent	619.7	588.0	630.8
Total Regulatory Assets	676.4	672.0	843.7
Regulatory Liabilities, Current	17.4	10.2	0.0
Regulatory Liabilities, Noncurrent	486.0	443.3	404.5
Total Regulatory Liabilities	503.4	453.5	404.5
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	17.4	19.2	31.0
Regulatory Assets, Noncurrent	647.3	642.0	612.6
Total Regulatory Assets	664.7	661.2	643.6
Regulatory Liabilities, Current	27.7	33.8	0.0
Regulatory Liabilities, Noncurrent	26.9	28.3	20.7
Total Regulatory Liabilities	54.6	62.1	20.7
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	12.2	28.9	207.0
Regulatory Assets, Noncurrent	0.0	1.2	20.3
Regulatory Liabilities, Current	39.2	42.3	0.0
Regulatory Liabilities, Noncurrent	40.4	17.2	23.7
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	11.8	28.1	194.1
Regulatory Assets, Noncurrent	0.0	1.2	20.3
Regulatory Liabilities, Current	17.4	10.2	0.0
Regulatory Liabilities, Noncurrent	40.4	17.2	23.7
Unamortized Purchased Gas Adjustments [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0.0	0.0	12.2
Regulatory Liabilities, Current	20.7	30.9	0.0
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	77.4	86.5	49.4
Regulatory Assets, Noncurrent	150.2	133.2	147.4
Regulatory Liabilities, Current	11.8	7.2	7.1
Regulatory Liabilities, Noncurrent	36.2	37.6	20.5
Other [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	44.9	55.9	18.8
Regulatory Assets, Noncurrent	137.2	119.6	133.2
Regulatory Liabilities, Noncurrent	32.2	33.2	15.6
Other [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	17.4	19.2	18.8
Regulatory Assets, Noncurrent	0.4	0.8	1.0
Regulatory Liabilities, Current	7.0	2.9	0.0
Regulatory Liabilities, Noncurrent	2.7	3.2	3.4
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	248.4	237.5	255.3
Regulatory Liabilities, Noncurrent	253.2	232.9	188.2
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	179.7	166.5	185.5
Regulatory Liabilities, Noncurrent	218.0	196.6	160.2
Pension and Postretirement Benefit Costs [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	66.1	68.2	66.0
Regulatory Liabilities, Noncurrent	24.2	25.1	17.3
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	676.1	668.2	640.8
Regulatory Liabilities, Noncurrent	136.7	133.6	130.3
Cost of Removal [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	97.0	97.0	97.0
Regulatory Liabilities, Noncurrent	96.8	94.5	93.4
Cost of Removal [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	579.1	571.2	543.7
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	152.4	150.7	145.6
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	144.5	142.7	137.3
Future Income Taxes Due from Customers [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	1.7	1.8	1.9
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	61.3	61.0	57.5
Energy Efficiency [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	61.3	61.0	57.5
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	110.7	114.2	124.5
Deferred Taxes Due to Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 98.6	$ 101.8	$ 111.6