Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 78.3	$ 115.4
Regulatory Assets, Noncurrent	1,323.5	1,251.8
Total Regulatory Assets	1,401.8	1,367.2
Regulatory Liabilities, Current	39.4	49.5
Regulatory Liabilities, Noncurrent	578.0	535.5
Total Regulatory Liabilities	617.4	585.0
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	48.9	84.0
Regulatory Assets, Noncurrent	653.2	588.0
Total Regulatory Assets	702.1	672.0
Regulatory Liabilities, Current	13.9	10.2
Regulatory Liabilities, Noncurrent	481.3	443.3
Total Regulatory Liabilities	495.2	453.5
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	16.1	19.2
Regulatory Assets, Noncurrent	650.6	642.0
Total Regulatory Assets	666.7	661.2
Regulatory Liabilities, Current	20.3	33.8
Regulatory Liabilities, Noncurrent	29.2	28.3
Total Regulatory Liabilities	49.5	62.1
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	154.2	150.7
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	146.6	142.7
Future Income Taxes Due from Customers [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	1.6	1.8
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	100.3	114.2
Deferred Taxes Due to Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	88.8	101.8
Deferred Taxes Due to Customers [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	0.0	0.0
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	217.8	237.5
Regulatory Liabilities, Noncurrent	300.1	232.9
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	157.0	166.5
Regulatory Liabilities, Noncurrent	261.2	196.6
Pension and Postretirement Benefit Costs [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	59.1	68.2
Regulatory Liabilities, Noncurrent	26.5	25.1
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	688.3	668.2
Regulatory Liabilities, Noncurrent	138.2	133.6
Cost of Removal [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	98.8	97.0
Regulatory Liabilities, Noncurrent	96.0	94.5
Cost of Removal [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	589.6	571.2
Regulatory Liabilities, Noncurrent	0.0	0.0
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	16.5	28.9
Regulatory Assets, Noncurrent	9.0	1.2
Regulatory Liabilities, Current	34.9	42.3
Regulatory Liabilities, Noncurrent	6.5	17.2
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	14.9	28.1
Regulatory Assets, Noncurrent	9.0	1.2
Regulatory Liabilities, Current	13.9	10.2
Regulatory Liabilities, Noncurrent	6.5	17.2
Unamortized Purchased Gas Adjustments [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0.0	0.0
Regulatory Assets, Noncurrent	0.0	0.0
Regulatory Liabilities, Current	20.2	30.9
Regulatory Liabilities, Noncurrent	0.0	0.0
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	65.0	61.0
Energy Efficiency [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	65.0	61.0
Energy Efficiency [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	0.0	0.0
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	61.8	86.5
Regulatory Assets, Noncurrent	189.2	133.2
Regulatory Liabilities, Current	4.5	7.2
Regulatory Liabilities, Noncurrent	32.9	37.6
Other [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	34.0	55.9
Regulatory Assets, Noncurrent	176.8	119.6
Regulatory Liabilities, Current	0.0	0.0
Regulatory Liabilities, Noncurrent	28.8	33.2
Other [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	16.1	19.2
Regulatory Assets, Noncurrent	0.3	0.8
Regulatory Liabilities, Current	0.1	2.9
Regulatory Liabilities, Noncurrent	$ 2.7	$ 3.2