Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 42.3	$ 115.4	$ 116.9
Regulatory Assets, Noncurrent	1,287.2	1,251.8	1,272.7
Total Regulatory Assets	1,329.5	1,367.2	1,389.6
Regulatory Liabilities, Current	51.6	49.5	25.1
Regulatory Liabilities, Noncurrent	637.0	535.5	557.7
Total Regulatory Liabilities	688.6	585.0	582.8
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.5	84.0	89.2
Regulatory Assets, Noncurrent	613.8	588.0	630.5
Total Regulatory Assets	629.3	672.0	719.7
Regulatory Liabilities, Current	12.0	10.2	0.0
Regulatory Liabilities, Noncurrent	546.1	443.3	475.3
Total Regulatory Liabilities	558.1	453.5	475.3
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	13.8	19.2	14.9
Regulatory Assets, Noncurrent	652.5	642.0	619.1
Total Regulatory Assets	666.3	661.2	634.0
Regulatory Liabilities, Current	33.9	33.8	18.7
Regulatory Liabilities, Noncurrent	26.4	28.3	20.1
Total Regulatory Liabilities	60.3	62.1	38.8
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0.3	28.9	81.4
Regulatory Assets, Noncurrent	0.0	1.2	0.0
Regulatory Liabilities, Current	46.3	42.3	19.8
Regulatory Liabilities, Noncurrent	100.2	17.2	90.8
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0.0	28.1	81.0
Regulatory Assets, Noncurrent	0.0	1.2	0.0
Regulatory Liabilities, Current	12.0	10.2	0.0
Regulatory Liabilities, Noncurrent	100.2	17.2	90.8
Unamortized Purchased Gas Adjustments [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Current	33.4	30.9	18.7
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	42.0	86.5	35.5
Regulatory Assets, Noncurrent	148.7	133.2	167.3
Regulatory Liabilities, Current	5.3	7.2	5.3
Regulatory Liabilities, Noncurrent	35.1	37.6	19.9
Other [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.5	55.9	8.2
Regulatory Assets, Noncurrent	135.8	119.6	153.2
Regulatory Liabilities, Noncurrent	31.2	33.2	15.0
Other [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	0.5	0.8	0.8
Regulatory Liabilities, Current	0.5	2.9	0.0
Regulatory Liabilities, Noncurrent	2.8	3.2	3.3
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	241.3	237.5	251.4
Regulatory Liabilities, Noncurrent	256.1	232.9	192.6
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	175.0	166.5	183.6
Regulatory Liabilities, Noncurrent	221.6	196.6	165.4
Pension and Postretirement Benefit Costs [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	63.9	68.2	64.2
Regulatory Liabilities, Noncurrent	23.6	25.1	16.8
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	679.9	668.2	649.2
Regulatory Liabilities, Noncurrent	138.5	133.6	133.5
Cost of Removal [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	93.5	97.0	97.0
Regulatory Liabilities, Noncurrent	97.8	94.5	95.8
Cost of Removal [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	586.4	571.2	552.2
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	153.7	150.7	146.6
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	145.9	142.7	138.5
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	63.6	61.0	58.2
Energy Efficiency [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	63.6	61.0	58.2
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	107.1	114.2	120.9
Deferred Taxes Due to Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 95.3	$ 101.8	$ 108.3