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	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Reported Value Measurement [Member]
Cash and cash equivalents	$ 4.8	$ 6.5	$ 8.2
Notes payable	1,227.0	1,037.5	846.0
Long-term debt, including current portion	3,412.9	3,239.7	3,238.0
Reported Value Measurement [Member] | Spire Missouri [Member]
Notes payable			250.0
Long-term debt, including current portion	1,638.1	1,637.7	1,637.0
Notes payable – associated companies	651.2	445.3	180.4
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents		2.4
Long-term debt, including current portion	745.6	571.5	571.3
Notes payable – associated companies	145.8	260.9	173.7
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	4.8	6.5	8.2
Notes payable	1,227.0	1,037.5	846.0
Long-term debt, including current portion	3,083.7	2,851.8	3,594.4
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Notes payable			250.0
Long-term debt, including current portion	1,497.7	1,473.9	1,830.5
Notes payable – associated companies	651.2	445.3	180.4
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Cash and cash equivalents		2.4
Long-term debt, including current portion	674.5	485.0	653.6
Notes payable – associated companies	145.8	260.9	173.7
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	4.8	6.5	8.2
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]
Notes payable			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		2.4
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	1,227.0	1,037.5	846.0
Long-term debt, including current portion	3,083.7	2,851.8	3,594.4
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Notes payable			250.0
Long-term debt, including current portion	1,497.7	1,473.9	1,830.5
Notes payable – associated companies	651.2	445.3	180.4
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	674.5	485.0	653.6
Notes payable – associated companies	$ 145.8	$ 260.9	$ 173.7