Summary of Significant Accounting Policies - Summary of Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | 12 Months Ended |
|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Sep. 30, 2025 |
|
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 81.2 | $ 71.8 | $ 82.5 |
| Spire Missouri [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | 46.3 | 52.9 | 61.8 |
| Spire Alabama Inc [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 4.7 | $ 9.2 | $ 9.5 |