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Financing Arrangements and Long-term Debt - Additional Information (Details)
1 Months Ended 3 Months Ended
Dec. 31, 2018
USD ($)
Oct. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Jan. 15, 2019
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 14, 2016
bank
Line of Credit Facility [Line Items]              
Line of credit facility borrowing capacity $ 975,000,000   $ 975,000,000        
Credit facility borrowings 0   0   $ 0 $ 0  
Non-interest-bearing Note              
Line of Credit Facility [Line Items]              
Settlement of debt   $ 10,000,000          
Term Loan              
Line of Credit Facility [Line Items]              
Long-term Debt $ 100,000,000   100,000,000        
Debt instrument, maturity date Dec. 01, 2021            
Liabilities | Lender Concentration Risk              
Line of Credit Facility [Line Items]              
Line of credit facility, number of banks in agreement | bank             11
Spire              
Line of Credit Facility [Line Items]              
Line of credit facility borrowing capacity $ 300,000,000   300,000,000        
Short-term borrowings used to support lending to Utilities 509,900,000   509,900,000        
Spire Missouri              
Line of Credit Facility [Line Items]              
Line of credit facility borrowing capacity 475,000,000   475,000,000        
Spire Alabama              
Line of Credit Facility [Line Items]              
Line of credit facility borrowing capacity $ 200,000,000   $ 200,000,000        
Spire Alabama | Senior Notes | Subsequent Event | Private Placement | Series 2019 A senior notes due 2049              
Line of Credit Facility [Line Items]              
Long-term debt       $ 90,000,000      
Stated interest rate       4.64%      
Line of Credit              
Line of Credit Facility [Line Items]              
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant 70.00%   70.00%        
Debt to total capitalization ratio 0.55   0.55        
Line of Credit | Spire Missouri              
Line of Credit Facility [Line Items]              
Debt to total capitalization ratio 0.50   0.50        
Line of Credit | Spire Alabama              
Line of Credit Facility [Line Items]              
Debt to total capitalization ratio 0.38   0.38        
Commercial Paper Notes              
Line of Credit Facility [Line Items]              
Long-term debt $ 975,000,000   $ 975,000,000        
Debt term     365 days