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Summary of Significant Accounting Policies - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |
Allowance at beginning of year | $ 23.0 | $ 22.4 | $ 18.3
Additions charged to expense | 14.0 | 16.9 | 14.6
Net deductions | (12.1) | (16.3) | (10.5)
Allowance at end of year | 24.9 | 23.0 | 22.4
Spire Missouri |  |  |
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |
Allowance at beginning of year | 14.9 | 16.0 | 14.1
Additions charged to expense | 12.7 | 12.3 | 11.9
Net deductions | (9.5) | (13.4) | (10.0)
Allowance at end of year | 18.1 | 14.9 | 16.0
Spire Alabama |  |  |
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |
Allowance at beginning of year | 6.3 | 3.9 | 2.6
Additions charged to expense | 0.9 | 4.7 | 2.1
Net deductions | (1.7) | (2.3) | (0.8)
Allowance at end of year | $ 5.5 | $ 6.3 | $ 3.9

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