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COMMITMENTS AND CONTINGENCIES - Leases and Guarantees (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Loss Contingencies [Line Items]      
Aggregate rental expense $ 9.7 $ 11.9 $ 14.1
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2017 10.1    
Minimum rental commitments - due in fiscal year 2018 9.3    
Minimum rental commitments - due in fiscal year 2019 7.8    
Minimum rental commitments - due in fiscal year 2020 6.1    
Minimum rental commitments - due in fiscal year 2021 6.0    
Minimum rental commitments - due later 44.4    
Minimum rental commitments - total 83.7    
Sublease rentals 2.1 2.1 0.2
Minimum future sublease rentals to be received - fiscal year 2017 2.1    
Minimum future sublease rentals to be received - fiscal year 2018 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    
Outstanding mortgage loans incurred in connection with various real estate ventures with general partners 1.3    
Spire Missouri      
Loss Contingencies [Line Items]      
Aggregate rental expense 4.8 4.3 7.6
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2017 2.1    
Minimum rental commitments - due in fiscal year 2018 1.3    
Minimum rental commitments - due in fiscal year 2019 0.2    
Minimum rental commitments - due in fiscal year 2020 0.2    
Minimum rental commitments - due in fiscal year 2021 0.0    
Minimum rental commitments - due later 0.0    
Minimum rental commitments - total 3.8    
Spire Alabama      
Loss Contingencies [Line Items]      
Aggregate rental expense 4.6 3.7 4.0
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
Minimum rental commitments - in fiscal year 2017 4.0    
Minimum rental commitments - due in fiscal year 2018 4.1    
Minimum rental commitments - due in fiscal year 2019 3.8    
Minimum rental commitments - due in fiscal year 2020 2.1    
Minimum rental commitments - due in fiscal year 2021 2.1    
Minimum rental commitments - due later 2.8    
Minimum rental commitments - total 18.9    
Sublease rentals 2.1 $ 2.1 $ 2.1
Minimum future sublease rentals to be received - fiscal year 2018 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