Fair Value of Financial Instruments - Schedule of Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 13.1	$ 4.5	$ 7.4
Notes payable	1,009.5	947.0	771.0
Long-term debt, including current portion	3,890.9	3,746.4	3,729.3
Reported Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			1.3
Notes payable – associated companies	398.7	495.3	418.3
Long-term debt, including current portion	1,953.2	1,803.4	1,786.6
Reported Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.7	1.5	2.3
Notes payable – associated companies	56.4	48.4	23.0
Long-term debt, including current portion	746.6	746.3	746.2
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.1	4.5	7.4
Notes payable	1,009.5	947.0	771.0
Long-term debt, including current portion	3,671.7	3,600.3	3,407.1
Estimate of Fair Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			1.3
Notes payable – associated companies	398.7	495.3	418.3
Long-term debt, including current portion	1,847.4	1,736.9	1,626.3
Estimate of Fair Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.7	1.5	2.3
Notes payable – associated companies	56.4	48.4	23.0
Long-term debt, including current portion	695.2	711.8	674.4
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.1	4.5	7.4
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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			1.3
Notes payable – associated companies	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 Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.7	1.5	2.3
Notes payable – associated companies	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 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Notes payable	1,009.5	947.0	771.0
Long-term debt, including current portion	3,671.7	3,600.3	3,407.1
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			0.0
Notes payable – associated companies	398.7	495.3	418.3
Long-term debt, including current portion	1,847.4	1,736.9	1,626.3
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Notes payable – associated companies	56.4	48.4	23.0
Long-term debt, including current portion	$ 695.2	$ 711.8	$ 674.4