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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Accounting Policies [Line Items]      
Allowance at beginning of year $ 31.4 $ 32.5 $ 31.9
Provision for expected credit losses 19.6 23.0 16.6
Write-offs, net of recoveries (22.2) (24.1) (16.0)
Allowance at end of year 28.8 31.4 32.5
Spire Missouri [Member]      
Accounting Policies [Line Items]      
Allowance at beginning of year 24.9 26.2 24.9
Provision for expected credit losses 14.8 19.0 13.3
Write-offs, net of recoveries (16.7) (20.3) (12.0)
Allowance at end of year 23.0 24.9 26.2
Spire Alabama Inc [Member]      
Accounting Policies [Line Items]      
Allowance at beginning of year 5.7 5.7 6.3
Provision for expected credit losses 4.0 3.2 2.6
Write-offs, net of recoveries (4.8) (3.2) (3.2)
Allowance at end of year $ 4.9 $ 5.7 $ 5.7