Segment Information - Schedule of Financial Data Related to Gross Receipt Taxes and Capital Expenditures (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
Mar. 31, 2026 |
Mar. 31, 2025 |
|
| Property, Plant and Equipment [Line Items] | ||||
| Capital Expenditures | $ 395.0 | $ 479.2 | ||
| Spire Missouri [Member] | ||||
| Property, Plant and Equipment [Line Items] | ||||
| Gross Receipt Taxes | $ 42.1 | $ 39.9 | 63.8 | 59.4 |
| Capital Expenditures | 135.2 | 143.8 | 283.2 | 322.1 |
| Spire Alabama Inc [Member] | ||||
| Property, Plant and Equipment [Line Items] | ||||
| Gross Receipt Taxes | 12.5 | 13.4 | 19.3 | 19.1 |
| Capital Expenditures | $ 45.1 | $ 28.6 | $ 86.4 | $ 63.1 |