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	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Reported Value Measurement [Member]
Cash and cash equivalents	$ 4.8	$ 5.6	$ 4.8
Notes payable	1,047.5	955.5	1,227.0
Long-term debt, including current portion	3,704.8	3,710.6	3,412.9
Reported Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Long-term debt, including current portion	1,785.8	1,785.4	1,638.1
Notes payable – associated companies	633.3	540.6	651.2
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents		1.2
Long-term debt, including current portion	746.0	745.9	745.6
Notes payable – associated companies	128.5	124.1	145.8
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	4.8	5.6	4.8
Notes payable	1,047.5	955.5	1,227.0
Long-term debt, including current portion	3,481.5	3,270.2	3,083.7
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Cash and cash equivalents		0.8
Long-term debt, including current portion	1,689.5	1,592.4	1,497.7
Notes payable – associated companies	633.3	540.6	651.2
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents		1.2
Long-term debt, including current portion	701.7	648.0	674.5
Notes payable – associated companies	128.5	124.1	145.8
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	4.8	5.6	4.8
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
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		1.2
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	1,047.5	955.5	1,227.0
Long-term debt, including current portion	3,481.5	3,270.2	3,083.7
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	1,689.5	1,592.4	1,497.7
Notes payable – associated companies	633.3	540.6	651.2
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents		0.0
Long-term debt, including current portion	701.7	648.0	674.5
Notes payable – associated companies	$ 128.5	$ 124.1	$ 145.8