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Financing - Summary of Net of Capitalized Interest (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020
Spire Missouri |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest expense, net | $ 0.1 | $ 0.3 | $ 0.1 | $ 0.6
Spire Alabama |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest expense, net | 0.7 | 0.4 | 1.3 | 1.0
Spire |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest expense, net | $ 1.1 | $ 1.4 | $ 2.0 | $ 3.6

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