Summary of Significant Accounting Policies - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Sep. 30, 2021 |
Sep. 30, 2020 |
Sep. 30, 2019 |
|
| Accounts Notes And Loans Receivable [Line Items] | |||
| Allowance at beginning of year | $ 24.9 | $ 23.0 | $ 22.4 |
| Provision for expected credit losses | 14.7 | 14.0 | 16.9 |
| Write-offs, net of recoveries | (9.3) | (12.1) | (16.3) |
| Allowance at end of year | 30.3 | 24.9 | 23.0 |
| Spire Missouri | |||
| Accounts Notes And Loans Receivable [Line Items] | |||
| Allowance at beginning of year | 18.1 | 14.9 | 16.0 |
| Provision for expected credit losses | 11.1 | 12.7 | 12.3 |
| Write-offs, net of recoveries | (6.6) | (9.5) | (13.4) |
| Allowance at end of year | 22.6 | 18.1 | 14.9 |
| Spire Alabama | |||
| Accounts Notes And Loans Receivable [Line Items] | |||
| Allowance at beginning of year | 5.5 | 6.3 | 3.9 |
| Provision for expected credit losses | 3.1 | 0.9 | 4.7 |
| Write-offs, net of recoveries | (2.0) | (1.7) | (2.3) |
| Allowance at end of year | $ 6.6 | $ 5.5 | $ 6.3 |