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COMMITMENTS AND CONTINGENCIES - Leases (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Loss Contingencies [Line Items]      
Aggregate rental expense $ 10.0 $ 9.7 $ 11.9
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2019 9.7    
Minimum rental commitments - due in fiscal year 2020 7.8    
Minimum rental commitments - due in fiscal year 2021 6.7    
Minimum rental commitments - due in fiscal year 2022 6.5    
Minimum rental commitments - due in fiscal year 2023 6.5    
Minimum rental commitments - due later 41.3    
Minimum rental commitments - total 78.5    
Sublease rentals 2.1 2.1 2.1
Minimum future sublease rentals to be received - fiscal year 2019 2.1    
Minimum future sublease rentals to be received - fiscal year 2020 0.5    
Spire Missouri      
Loss Contingencies [Line Items]      
Aggregate rental expense 3.6 4.8 4.3
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2019 1.3    
Minimum rental commitments - due in fiscal year 2020 0.6    
Minimum rental commitments - due in fiscal year 2021 0.3    
Minimum rental commitments - due in fiscal year 2022 0.1    
Minimum rental commitments - due in fiscal year 2023 0.0    
Minimum rental commitments - due later 0.0    
Minimum rental commitments - total 2.3    
Spire Alabama      
Loss Contingencies [Line Items]      
Aggregate rental expense 4.7 4.6 3.7
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2019 4.1    
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 later 0.7    
Minimum rental commitments - total 14.0    
Sublease rentals 2.1 $ 2.1 $ 2.1
Minimum future sublease rentals to be received - fiscal year 2019 2.1    
Minimum future sublease rentals to be received - fiscal year 2020 $ 0.5