Note 8 - 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, 2023	Sep. 30, 2022
Reported Value Measurement [Member]
Cash and cash equivalents	$ 5.6	$ 6.5
Notes payable	955.5	1,037.5
Long-term debt, including current portion	3,710.6	3,239.7
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents	0.8
Long-term debt, including current portion	1,785.4	1,637.7
Notes payable - associated companies	540.6	445.3
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.2	2.4
Long-term debt, including current portion	745.9	571.5
Notes payable - associated companies	124.1	260.9
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	5.6	6.5
Notes payable	955.5	1,037.5
Long-term debt, including current portion	3,270.2	2,851.8
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents	0.8
Long-term debt, including current portion	1,592.4	1,473.9
Notes payable - associated companies	540.6	445.3
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	1.2	2.4
Long-term debt, including current portion	648.0	485.0
Notes payable - associated companies	124.1	260.9
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	5.6	6.5
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	1.2	2.4
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	955.5	1,037.5
Long-term debt, including current portion	3,270.2	2,851.8
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	1,592.4	1,473.9
Notes payable - associated companies	540.6	445.3
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	648.0	485.0
Notes payable - associated companies	$ 124.1	$ 260.9