Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Accounting Policies [Line Items] | ||||
| Allowance at beginning of period | $ 36.8 | $ 39.1 | $ 31.4 | $ 32.5 |
| Provision for expected credit losses | 3.6 | 5.6 | 17.6 | 17.6 |
| Write-offs, net of recoveries | (6.6) | (11.5) | (15.2) | (16.9) |
| Allowance at end of period | 33.8 | 33.2 | 33.8 | 33.2 |
| Spire Missouri [Member] | ||||
| Accounting Policies [Line Items] | ||||
| Allowance at beginning of period | 29.6 | 32.0 | 24.9 | 26.2 |
| Provision for expected credit losses | 2.6 | 4.7 | 13.3 | 14.5 |
| Write-offs, net of recoveries | (4.3) | (10.1) | (10.3) | (14.1) |
| Allowance at end of period | 27.9 | 26.6 | 27.9 | 26.6 |
| Spire Alabama Inc [Member] | ||||
| Accounting Policies [Line Items] | ||||
| Allowance at beginning of period | 6.1 | 6.2 | 5.7 | 5.7 |
| Provision for expected credit losses | 0.7 | 0.7 | 3.5 | 2.4 |
| Write-offs, net of recoveries | (1.7) | (1.2) | (4.1) | (2.4) |
| Allowance at end of period | $ 5.1 | $ 5.7 | $ 5.1 | $ 5.7 |