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Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020
Spire Marketing Inc | Spire Missouri | Regulated Operation |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Purchases of natural gas from related party | $ 57.6 | $ 13.3 | $ 68.3 | $ 32.8
Sales of natural gas to Spire Marketing Inc. |  |  | 1.1 |
Spire Marketing Inc | Spire Alabama | Regulated Operation |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Purchases of natural gas from related party | 5.8 | 1.0 | 10.6 | 4.3
Sales of natural gas to Spire Marketing Inc. | 0.1 |  | 0.1 |
Spire STL Pipeline LLC | Spire Missouri | Unregulated Operation |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Transportation services received | 7.9 | 7.9 | 16.0 | 11.8
Spire NGL Inc. | Spire Missouri | Unregulated Operation |  |  |  |
Related Party Transaction [Line Items] |  |  |  |
Transportation services received | $ 0.2 | $ 0.2 | $ 0.5 | $ 0.5

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