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	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Reported Value Measurement [Member]
Cash and cash equivalents	$ 5.3	$ 6.5	$ 16.0
Notes payable	557.6	1,037.5	709.2
Long-term debt, including current portion	3,959.9	3,239.7	3,239.1
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents			7.0
Long-term debt, including current portion	2,035.1	1,637.7	1,637.4
Notes payable – associated companies	228.1	445.3	285.4
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	0.5
Long-term debt, including current portion	745.8	571.5	571.4
Notes payable – associated companies	108.1	260.9	196.3
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	5.3	6.5	16.0
Notes payable	557.6	1,037.5	709.2
Long-term debt, including current portion	3,650.7	2,851.8	3,032.1
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents			7.0
Long-term debt, including current portion	1,890.2	1,473.9	1,562.4
Notes payable – associated companies	228.1	445.3	285.4
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	0.5	2.4	3.7
Long-term debt, including current portion	681.5	485.0	527.2
Notes payable – associated companies	108.1	260.9	196.3
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	5.3	6.5	16.0
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			7.0
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	0.5	2.4	3.7
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	557.6	1,037.5	709.2
Long-term debt, including current portion	3,650.7	2,851.8	3,032.1
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	1,890.2	1,473.9	1,562.4
Notes payable – associated companies	228.1	445.3	285.4
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	681.5	485.0	527.2
Notes payable – associated companies	$ 108.1	$ 260.9	$ 196.3