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	Mar. 31, 2026	Sep. 30, 2025	Mar. 31, 2025
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 49.5	$ 5.7	$ 15.2
Notes payable	1,955.0	1,317.0	1,015.0
Long-term debt, including current portion	6,000.1	3,856.9	3,741.0
Reported Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	381.1	566.3	461.3
Long-term debt, including current portion	2,153.1	1,953.6	1,803.8
Reported Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	1.9	3.3
Notes payable - associated companies	149.8	130.1	41.8
Long-term debt, including current portion	711.9	711.7	746.5
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	49.5	5.7	15.2
Notes payable	1,955.0	1,317.0	1,015.0
Long-term debt, including current portion	5,849.3	3,691.5	3,481.2
Estimate of Fair Value Measurement [Member] \| Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	381.1	566.3	461.3
Long-term debt, including current portion	2,041.0	1,874.0	1,671.5
Estimate of Fair Value Measurement [Member] \| Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	1.9	3.3
Notes payable - associated companies	149.8	130.1	41.8
Long-term debt, including current portion	658.7	675.9	687.8
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	49.5	5.7	15.2
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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	381.1	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	2.0	1.9	3.3
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	1,955.0	1,317.0	1,015.0
Long-term debt, including current portion	5,849.3	3,691.5	3,481.2
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	0.0	566.3	461.3
Long-term debt, including current portion	2,041.0	1,874.0	1,671.5
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	149.8	130.1	41.8
Long-term debt, including current portion	$ 658.7	$ 675.9	$ 687.8