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, 2024	Sep. 30, 2023	Mar. 31, 2023
Reported Value Measurement [Member]
Cash and cash equivalents	$ 25.6	$ 5.6	$ 6.9
Notes payable	786.0	955.5	561.0
Long-term debt, including current portion	3,728.4	3,710.6	3,959.1
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Notes payable	200.0		200.0
Long-term debt, including current portion	1,786.2	1,785.4	2,034.5
Notes payable – associated companies	243.9	540.6	60.7
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.8	1.2	1.0
Long-term debt, including current portion	746.1	745.9	745.7
Notes payable – associated companies	37.4	124.1	121.3
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	25.6	5.6	6.9
Notes payable	786.0	955.5	561.0
Long-term debt, including current portion	3,452.0	3,270.2	3,704.2
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Notes payable	200.0		200.0
Long-term debt, including current portion	1,649.4	1,592.4	1,930.2
Notes payable – associated companies	243.9	540.6	60.7
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.8	1.2	1.0
Long-term debt, including current portion	685.3	648.0	696.6
Notes payable – associated companies	37.4	124.1	121.3
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	25.6	5.6	6.9
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]
Cash and cash equivalents		0.8
Notes payable	0.0		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	1.8	1.2	1.0
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 2 [Member]
Cash and cash equivalents	0.0	0.0	0.0
Notes payable	786.0	955.5	561.0
Long-term debt, including current portion	3,452.0	3,270.2	3,704.2
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.0
Notes payable	200.0		200.0
Long-term debt, including current portion	1,649.4	1,592.4	1,930.2
Notes payable – associated companies	243.9	540.6	60.7
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	685.3	648.0	696.6
Notes payable – associated companies	$ 37.4	$ 124.1	$ 121.3