Note 7 - Fair Value of Financial Instruments - Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022	Mar. 31, 2022
Reported Value Measurement [Member]
Cash and cash equivalents	$ 6.9	$ 6.5	$ 8.3
Notes payable	561.0	1,037.5	607.1
Long-term debt, including current portion	3,959.1	3,239.7	3,238.5
Reported Value Measurement [Member] | Spire Missouri [Member]
Notes payable	200.0
Long-term debt, including current portion	2,034.5	1,637.7	1,637.1
Notes payable – associated companies	60.7	445.3	277.2
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.0	2.4
Long-term debt, including current portion	745.7	571.5	571.4
Notes payable – associated companies	121.3	260.9	156.6
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	6.9	6.5	8.3
Notes payable	561.0	1,037.5	607.1
Long-term debt, including current portion	3,704.2	2,851.8	3,299.1
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Notes payable	200.0
Long-term debt, including current portion	1,930.2	1,473.9	1,691.9
Notes payable – associated companies	60.7	445.3	277.2
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.0	2.4	0.2
Long-term debt, including current portion	696.6	485.0	588.0
Notes payable – associated companies	121.3	260.9	156.6
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	6.9	6.5	8.3
Notes payable	0.0	0.0	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Notes payable	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Notes payable – associated companies	0.0	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.0	2.4	0.2
Long-term debt, including current portion	0.0	0.0	0.0
Notes payable – associated companies	0.0	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0.0	0.0	0.0
Notes payable	561.0	1,037.5	607.1
Long-term debt, including current portion	3,704.2	2,851.8	3,299.1
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Notes payable	200.0
Long-term debt, including current portion	1,930.2	1,473.9	1,691.9
Notes payable – associated companies	60.7	445.3	277.2
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	0.0	0.0	0.0
Long-term debt, including current portion	696.6	485.0	588.0
Notes payable – associated companies	$ 121.3	$ 260.9	$ 156.6