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	Sep. 30, 2024	Sep. 30, 2023
Reported Value Measurement [Member]
Cash and cash equivalents	$ 4.5	$ 5.6
Notes payable	947.0	955.5
Long-term debt, including current portion	3,746.4	3,710.6
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Long-term debt, including current portion	1,803.4	1,785.4
Notes payable - associated companies	495.3	540.6
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.5	1.2
Long-term debt, including current portion	746.3	745.9
Notes payable - associated companies	48.4	124.1
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	4.5	5.6
Notes payable	947.0	955.5
Long-term debt, including current portion	3,600.3	3,270.2
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Long-term debt, including current portion	1,736.9	1,592.4
Notes payable - associated companies	495.3	540.6
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.5	1.2
Long-term debt, including current portion	711.8	648.0
Notes payable - associated companies	48.4	124.1
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	4.5	5.6
Notes payable	0.0	0.0
Long-term debt, including current portion	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
Long-term debt, including current portion	0.0	0.0
Notes payable - associated companies	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.5	1.2
Long-term debt, including current portion	0.0	0.0
Notes payable - associated companies	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0.0	0.0
Notes payable	947.0	955.5
Long-term debt, including current portion	3,600.3	3,270.2
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.0
Long-term debt, including current portion	1,736.9	1,592.4
Notes payable - associated companies	495.3	540.6
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	0.0	0.0
Long-term debt, including current portion	711.8	648.0
Notes payable - associated companies	$ 48.4	$ 124.1