Summary of Significant Accounting Policies - Summary of Accrued Capital Expenditures Excluded from Capital Expenditures in the Statements of Cash Flows (Details) - USD ($) $ in Millions |
6 Months Ended | 12 Months Ended | |
|---|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
Sep. 30, 2025 |
|
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 54.9 | $ 66.8 | $ 82.5 |
| Spire Missouri [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | 44.2 | 45.6 | 61.8 |
| Spire Alabama Inc [Member] | |||
| Accounting Policies [Line Items] | |||
| Accrued capital expenditures | $ 5.8 | $ 8.5 | $ 9.5 |