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Summary of Significant Accounting Policies (Tables) | 6 Months Ended
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Mar. 31, 2021
Accounting Policies [Abstract] |
Schedule of Inter-Company Transactions | Spire Missouri’s and Spire Alabama’s other transactions with affiliates are presented below
|  | Three Months Ended March 31, |  |  | Six Months Ended March 31, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of natural gas from Spire Marketing Inc. |  | $ | 57.6 |  |  | $ | 13.3 |  |  | $ | 68.3 |  |  | $ | 32.8 |
Transportation services received from Spire STL Pipeline LLC |  |  | 7.9 |  |  |  | 7.9 |  |  |  | 16.0 |  |  |  | 11.8 |
Sales of natural gas to Spire Marketing Inc. |  |  | — |  |  |  | — |  |  |  | 1.1 |  |  |  | — |
Transportation services received from Spire NGL Inc. |  |  | 0.2 |  |  |  | 0.2 |  |  |  | 0.5 |  |  |  | 0.5 |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases of natural gas from Spire Marketing Inc. |  | $ | 5.8 |  |  | $ | 1.0 |  |  | $ | 10.6 |  |  | $ | 4.3 |
Sales of natural gas to Spire Marketing Inc. |  |  | 0.1 |  |  |  | — |  |  |  | 0.1 |  |  |  | — |
Schedule of Capital Expenditures Excluded from Statement of Cash Flow | Accrued capital expenditures, shown in the following table, are excluded from capital expenditures in the statements of cash flows until paid.
|  | March 31, |  |  | September 30, |  |  | March 31, |
|  | 2021 |  |  | 2020 |  |  | 2020 |
Spire |  | $ | 36.1 |  |  | $ | 67.6 |  |  | $ | 51.4 |
Spire Missouri |  |  | 21.8 |  |  |  | 34.3 |  |  |  | 21.5 |
Spire Alabama |  |  | 4.7 |  |  |  | 17.0 |  |  |  | 17.6 |
Schedule of Allowance for Credit Losses |
| Three Months Ended March 31, 2021 |  |  | Six Months Ended March 31, 2021 |
|  |  |  |  | Spire |  |  | Spire |  |  |  |  |  |  | Spire |  |  | Spire |
| Spire |  |  | Missouri |  |  | Alabama |  |  | Spire |  |  | Missouri |  |  | Alabama |
Allowance at beginning of period | $ | 28.9 |  |  | $ | 21.4 |  |  | $ | 6.2 |  |  | $ | 24.9 |  |  | $ | 18.1 |  |  | $ | 5.5 |
Provision for expected credit losses |  | 6.6 |  |  |  | 5.7 |  |  |  | 0.6 |  |  |  | 10.7 |  |  |  | 9.0 |  |  |  | 1.3 |
Write-offs, net of recoveries |  | (1.1 | ) |  |  | (0.5 | ) |  |  | (0.3 | ) |  |  | (1.2 | ) |  |  | (0.5 | ) |  |  | (0.3 | )
Allowance at end of period | $ | 34.4 |  |  | $ | 26.6 |  |  | $ | 6.5 |  |  | $ | 34.4 |  |  | $ | 26.6 |  |  | $ | 6.5 |

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