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Long-term Debt - Additional Information (Details) - USD ($)
12 Months Ended
Nov. 12, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Jul. 09, 2019
Debt Instrument [Line Items]          
Long-term debt   $ 2,137,000,000.0 $ 2,092,000,000.0    
Interest expense, net   6,800,000 2,600,000 $ 800,000  
Spire Missouri          
Debt Instrument [Line Items]          
Long-term debt   930,000,000.0 880,000,000.0    
Interest expense, net   1,900,000 900,000 $ 500,000  
Regulatory preferred stock and debt securities issuance, amount authorized   500,000,000.0      
Regulatory preferred stock and debt securities issuance, remaining authorized amount   400,000,000.0      
Spire Missouri | Floating-rate note due December 1, 2021          
Debt Instrument [Line Items]          
Long-term debt   100,000,000.0      
Spire Missouri | Floating-rate note due December 1, 2021 | Subsequent Event          
Debt Instrument [Line Items]          
Repayment of long-term debt $ 100,000,000.0        
Spire Missouri | 2.84% Series, due November 15, 2029 | Subsequent Event          
Debt Instrument [Line Items]          
Long-term debt $ 275,000,000.0        
Spire Alabama          
Debt Instrument [Line Items]          
Long-term debt   415,000,000.0 $ 325,000,000.0    
Regulatory preferred stock and debt securities issuance, amount authorized         $ 100,000,000.0
Subsidiaries          
Debt Instrument [Line Items]          
Long-term debt   $ 102,000,000.0