Note 6 - 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, 2022	Sep. 30, 2021	Jun. 30, 2021
Reported Value Measurement [Member]
Cash and cash equivalents	$ 16.0	$ 4.3	$ 23.9
Notes payable	709.2	672.0	461.0
Long-term debt, including current portion	3,239.1	2,994.9	3,049.8
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents	7.0		4.3
Notes payable		250.0	250.0
Long-term debt, including current portion	1,637.4	1,338.4	1,338.6
Notes payable – associated companies	285.4	240.9	217.5
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	3.7		9.5
Long-term debt, including current portion	571.4	621.2	621.2
Notes payable – associated companies	196.3	49.0
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	16.0	4.3	23.9
Notes payable	709.2	672.0	461.0
Long-term debt, including current portion	3,032.1	3,375.9	3,440.2
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents	7.0		4.3
Notes payable		250.0	250.0
Long-term debt, including current portion	1,562.4	1,540.4	1,548.8
Notes payable – associated companies	285.4	240.9	217.5
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents	3.7		9.5
Long-term debt, including current portion	527.2	707.5	710.5
Notes payable – associated companies	196.3	49.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	16.0	4.3	23.9
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	7.0		4.3
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	3.7		9.5
Long-term debt, including current portion	0.0	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	0.0
Notes payable	709.2	672.0	461.0
Long-term debt, including current portion	3,032.1	3,375.9	3,440.2
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Cash and cash equivalents	0.0		0.0
Notes payable		250.0	250.0
Long-term debt, including current portion	1,562.4	1,540.4	1,548.8
Notes payable – associated companies	285.4	240.9	217.5
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	527.2	707.5	$ 710.5
Notes payable – associated companies	$ 196.3	$ 49.0