Summary of Significant Accounting Policies - Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended | |
|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Sep. 30, 2024 |
|
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 71.8 | $ 81.6 | $ 116.5 |
| Spire Missouri [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | 52.9 | 43.5 | 67.4 |
| Spire Alabama Inc [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 9.2 | $ 5.5 | $ 14.1 |