Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 63.4	$ 78.3	$ 89.6
Regulatory Assets, Noncurrent	1,331.4	1,323.5	1,288.4
Total Regulatory Assets	1,394.8	1,401.8	1,378.0
Regulatory Liabilities, Current	34.4	39.4	51.0
Regulatory Liabilities, Noncurrent	587.6	578.0	577.2
Total Regulatory Liabilities	622.0	617.4	628.2
Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	34.8	48.9	56.7
Regulatory Assets, Noncurrent	655.1	653.2	619.7
Total Regulatory Assets	689.9	702.1	676.4
Regulatory Liabilities, Current	10.5	13.9	17.4
Regulatory Liabilities, Noncurrent	490.9	481.3	486.0
Total Regulatory Liabilities	501.4	495.2	503.4
Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.0	16.1	17.4	$ 16.1
Regulatory Assets, Noncurrent	657.0	650.6	647.3	$ 650.6
Total Regulatory Assets	672.0	666.7	664.7
Regulatory Liabilities, Current	18.2	20.3	27.7
Regulatory Liabilities, Noncurrent	28.8	29.2	26.9
Total Regulatory Liabilities	47.0	49.5	54.6
Unamortized Purchased Gas Adjustments [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	10.1	16.5	12.2
Regulatory Assets, Noncurrent	4.6	9.0	0.0
Regulatory Liabilities, Current	29.3	34.9	39.2
Regulatory Liabilities, Noncurrent	17.9	6.5	40.4
Unamortized Purchased Gas Adjustments [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	8.5	14.9	11.8
Regulatory Assets, Noncurrent	4.6	9.0	0.0
Regulatory Liabilities, Current	10.5	13.9	17.4
Regulatory Liabilities, Noncurrent	17.9	6.5	40.4
Unamortized Purchased Gas Adjustments [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Current	18.2	20.2	20.7
Other [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	53.3	61.8	77.4
Regulatory Assets, Noncurrent	196.2	189.2	150.2
Regulatory Liabilities, Current	5.1	4.5	11.8
Regulatory Liabilities, Noncurrent	32.2	32.9	36.2
Other [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	26.3	34.0	44.9
Regulatory Assets, Noncurrent	184.2	176.8	137.2
Regulatory Liabilities, Noncurrent	28.2	28.8	32.2
Other [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	15.0	16.1	17.4
Regulatory Assets, Noncurrent	0.4	0.3	0.4
Regulatory Liabilities, Current	0.0	0.1	7.0
Regulatory Liabilities, Noncurrent	2.6	2.7	2.7
Pension and Postretirement Benefit Costs [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	212.2	217.8	248.4
Regulatory Liabilities, Noncurrent	301.8	300.1	253.2
Pension and Postretirement Benefit Costs [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	152.6	157.0	179.7
Regulatory Liabilities, Noncurrent	263.3	261.2	218.0
Pension and Postretirement Benefit Costs [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	58.1	59.1	66.1
Regulatory Liabilities, Noncurrent	26.2	26.5	24.2
Cost of Removal [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	698.4	688.3	676.1
Regulatory Liabilities, Noncurrent	138.9	138.2	136.7
Cost of Removal [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	101.3	98.8	97.0
Regulatory Liabilities, Noncurrent	96.0	96.0	96.8
Cost of Removal [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	597.0	589.6	579.1
Future Income Taxes Due from Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	154.5	154.2	152.4
Future Income Taxes Due from Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	146.9	146.6	144.5
Future Income Taxes Due from Customers [Member] | Spire Alabama Inc [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	1.5	1.6	1.7
Energy Efficiency [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	65.5	65.0	61.3
Energy Efficiency [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Noncurrent	65.5	65.0	61.3
Deferred Taxes Due to Customers [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	96.8	100.3	110.7
Deferred Taxes Due to Customers [Member] | Spire Missouri [Member]
Schedule Of Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities, Noncurrent	$ 85.5	$ 88.8	$ 98.6