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Summary of Significant Accounting Policies - Schedule of Inter-Company Transactions (Details) - Affiliated Entity - Spire Missouri - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Spire Marketing | Regulated Operation |  |  |
Related Party Transaction [Line Items] |  |  |
Purchases of natural gas from related party | $ 56.9 | $ 95.3 | $ 71.5
Sales of natural gas to Spire Marketing Inc. | 0.1 | 1.7 | 0.3
Spire STL Pipeline LLC | Unregulated Operation |  |  |
Related Party Transaction [Line Items] |  |  |
Transportation services received | 27.9 |  |
Spire NGL Inc. | Unregulated Operation |  |  |
Related Party Transaction [Line Items] |  |  |
Transportation services received | $ 1.0 | $ 1.0 | $ 1.0

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