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	Sep. 30, 2025	Sep. 30, 2024
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 5.7	$ 4.5
Notes payable	1,317.0	947.0
Long-term debt, including current portion	3,856.9	3,746.4
Reported Value Measurement [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	566.3	495.3
Long-term debt, including current portion	1,953.6	1,803.4
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.9	1.5
Notes payable - associated companies	130.1	48.4
Long-term debt, including current portion	711.7	746.3
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.7	4.5
Notes payable	1,317.0	947.0
Long-term debt, including current portion	3,691.5	3,600.3
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	566.3	495.3
Long-term debt, including current portion	1,874.0	1,736.9
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.9	1.5
Notes payable - associated companies	130.1	48.4
Long-term debt, including current portion	675.9	711.8
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	5.7	4.5
Notes payable	0.0	0.0
Long-term debt, including current portion	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
Long-term debt, including current portion	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	1.9	1.5
Notes payable - associated companies	0.0	0.0
Long-term debt, including current portion	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
Notes payable	1,317.0	947.0
Long-term debt, including current portion	3,691.5	3,600.3
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	566.3	495.3
Long-term debt, including current portion	1,874.0	1,736.9
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
Notes payable - associated companies	130.1	48.4
Long-term debt, including current portion	$ 675.9	$ 711.8