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, 2026	Sep. 30, 2025	Jun. 30, 2025
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 20.7	$ 5.7	$ 13.1
Notes payable	1,263.5	1,317.0	1,009.5
Long-term debt, including current portion	5,996.1	3,856.9	3,890.9
Reported Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	443.5	566.3	398.7
Long-term debt, including current portion	2,153.6	1,953.6	1,953.2
Reported Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.8	1.9	1.7
Notes payable - associated companies	168.2	130.1	56.4
Long-term debt, including current portion	712.0	711.7	746.6
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	20.7	5.7	13.1
Notes payable	1,263.5	1,317.0	1,009.5
Long-term debt, including current portion	5,839.9	3,691.5	3,671.7
Estimate of Fair Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	443.5	566.3	398.7
Long-term debt, including current portion	2,037.4	1,874.0	1,847.4
Estimate of Fair Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.8	1.9	1.7
Notes payable - associated companies	168.2	130.1	56.4
Long-term debt, including current portion	659.2	675.9	695.2
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	20.7	5.7	13.1
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]
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	2.8	1.9	1.7
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	1,263.5	1,317.0	1,009.5
Long-term debt, including current portion	5,839.9	3,691.5	3,671.7
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]
Notes payable - associated companies	443.5	566.3	398.7
Long-term debt, including current portion	2,037.4	1,874.0	1,847.4
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	168.2	130.1	56.4
Long-term debt, including current portion	$ 659.2	$ 675.9	$ 695.2