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Commitments and Contingencies - Aggregate Rental Expense and Annual Minimum Rental Commitments (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Loss Contingencies [Line Items]      
Aggregate Rental Expense $ 10.9 $ 10.0 $ 9.7
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum Rental Commitments - due in fiscal year 2020 8.2    
Minimum Rental Commitments - due in fiscal year 2021 7.0    
Minimum Rental Commitments - due in fiscal year 2022 6.8    
Minimum Rental Commitments - due in fiscal year 2023 6.1    
Minimum Rental Commitments - due in fiscal year 2024 4.8    
Minimum Rental Commitments - due later 36.5    
Minimum Rental Commitments - total 69.4    
Spire Missouri      
Loss Contingencies [Line Items]      
Aggregate Rental Expense 3.7 3.6 4.8
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum Rental Commitments - due in fiscal year 2020 0.5    
Minimum Rental Commitments - due in fiscal year 2021 0.2    
Minimum Rental Commitments - due in fiscal year 2023 0.0    
Minimum Rental Commitments - due later 0.0    
Minimum Rental Commitments - total 0.7    
Spire Alabama      
Loss Contingencies [Line Items]      
Aggregate Rental Expense 5.2 $ 4.7 $ 4.6
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum Rental Commitments - due in fiscal year 2020 2.9    
Minimum Rental Commitments - due in fiscal year 2021 2.1    
Minimum Rental Commitments - due in fiscal year 2022 2.1    
Minimum Rental Commitments - due in fiscal year 2023 2.1    
Minimum Rental Commitments - due in fiscal year 2024 0.7    
Minimum Rental Commitments - total $ 9.9