Note 15 - Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Regulatory assets	$ 348.3	$ 355.4
Regulatory Assets Noncurrent	1,249.2	1,112.4
Total Regulatory Assets	1,597.5	1,467.8
Regulatory Liability Current	7.3	3.7
Regulatory Liability Noncurrent	472.4	418.2
Total Regulatory Liabilities	479.7	421.9
Spire Missouri [Member]
Regulatory assets	293.1	288.1
Regulatory Assets Noncurrent	617.6	547.6
Total Regulatory Assets	910.7	835.7
Regulatory Liability Current	0.0	0.0
Regulatory Liability Noncurrent	389.4	331.8
Total Regulatory Liabilities	389.4	331.8
Spire Alabama Inc [Member]
Regulatory assets	41.6	56.9
Regulatory Assets Noncurrent	606.9	538.2
Total Regulatory Assets	648.5	595.1
Regulatory Liability Current	0.0	0.0
Regulatory Liability Noncurrent	21.2	23.0
Total Regulatory Liabilities	21.2	23.0
Unamortized Purchased Gas Adjustments Asset [Member]
Regulatory assets	293.2	322.2
Regulatory Assets Noncurrent	23.0	0.0
Regulatory Liability Current	11.2	53.0
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Missouri [Member]
Regulatory assets	269.4	275.1
Regulatory Assets Noncurrent	23.0	0.0
Regulatory Liability Current	11.2	53.0
Unamortized Purchased Gas Adjustments Asset [Member] | Spire Alabama Inc [Member]
Regulatory assets	21.7	43.8
Regulatory Assets Noncurrent	0.0	0.0
Regulatory Liability Current	0.0	0.0
Other Regulatory Assets (Liabilities) [Member]
Regulatory assets	55.1	33.2
Regulatory Assets Noncurrent	131.2	129.2
Regulatory Liability Current	7.3	3.7
Regulatory Liability Noncurrent	21.4	14.4
Other Regulatory Assets (Liabilities) [Member] | Spire Missouri [Member]
Regulatory assets	23.7	13.0
Regulatory Assets Noncurrent	116.0	113.1
Regulatory Liability Current	0.0	0.0
Regulatory Liability Noncurrent	16.5	7.3
Other Regulatory Assets (Liabilities) [Member] | Spire Alabama Inc [Member]
Regulatory assets	19.9	13.1
Regulatory Assets Noncurrent	0.9	1.0
Regulatory Liability Current	0.0	0.0
Regulatory Liability Noncurrent	3.3	3.6
Deferred Income Tax Charge [Member]
Regulatory Assets Noncurrent	144.5	137.8
Deferred Income Tax Charge [Member] | Spire Missouri [Member]
Regulatory Assets Noncurrent	136.2	129.2
Deferred Income Tax Charge [Member] | Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	2.0	2.2
Pension and Other Postretirement Plans Costs [Member]
Regulatory assets	261.0	294.5
Regulatory Assets Noncurrent	185.2	172.6
Pension and Other Postretirement Plans Costs [Member] | Spire Missouri [Member]
Regulatory assets	189.1	222.9
Regulatory Assets Noncurrent	156.5	143.6
Pension and Other Postretirement Plans Costs [Member] | Spire Alabama Inc [Member]
Regulatory assets	67.8	66.5
Regulatory Assets Noncurrent	17.9	19.4
Removal Costs [Member]
Regulatory Assets Noncurrent	633.2	493.7
Regulatory Liability Noncurrent	126.6	32.9
Removal Costs [Member] | Spire Missouri [Member]
Regulatory Assets Noncurrent	97.0	25.2
Regulatory Liability Noncurrent	90.4	0.0
Removal Costs [Member] | Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	536.2	468.5
Regulatory Liability Noncurrent	0.0	0.0
Renewable Energy Program [Member]
Regulatory Assets Noncurrent	56.3	57.2
Renewable Energy Program [Member] | Spire Missouri [Member]
Regulatory Assets Noncurrent	56.3	57.2
Renewable Energy Program [Member] | Spire Alabama Inc [Member]
Regulatory Assets Noncurrent	0.0	0.0
Estimated Deferred Taxes Due to Customers [Member]
Regulatory Liability Noncurrent	128.0	145.3
Estimated Deferred Taxes Due to Customers [Member] | Spire Missouri [Member]
Regulatory Liability Noncurrent	114.8	127.9
Estimated Deferred Taxes Due to Customers [Member] | Spire Alabama Inc [Member]
Regulatory Liability Noncurrent	$ 0.0	$ 0.0