v3.25.2
Financing - Additional Information (Details) - USD ($)
9 Months Ended
May 01, 2025
May 06, 2024
Apr. 05, 2024
Jan. 03, 2024
Jun. 30, 2025
Sep. 30, 2024
Jun. 30, 2024
Short-Term Debt [Line Items]              
Notes outstanding         $ 1,009,500,000 $ 947,000,000 $ 771,000,000
Parent Company [Member] | First Mortgage Bond [Member]              
Short-Term Debt [Line Items]              
Debt Instrument face amount $ 150,000,000            
Interest payable term         semi-annually on March 15 and September 15 of each year    
Parent Company [Member] | First Mortgage Bond [Member] | First Tranche [Member]              
Short-Term Debt [Line Items]              
Debt Instrument face amount $ 90,000,000            
Debt instrument interest rate stated percentage 4.88%            
Maturity Date Sep. 15, 2030            
Parent Company [Member] | First Mortgage Bond [Member] | Second Tranche [member]              
Short-Term Debt [Line Items]              
Debt Instrument face amount $ 60,000,000            
Debt instrument interest rate stated percentage 5.12%            
Maturity Date Sep. 15, 2032            
Commercial Paper [Member] | Parent Company [Member]              
Short-Term Debt [Line Items]              
Notes outstanding         $ 1,009,500,000 $ 947,000,000 $ 771,000,000
Short term borrowings used to support lending to utilities         486,600,000    
Maximum [Member] | Commercial Paper [Member]              
Short-Term Debt [Line Items]              
Notes outstanding         1,500,000,000    
Spire Missouri [Member] | Loans Payable [Member]              
Short-Term Debt [Line Items]              
Debt Instrument face amount       $ 200,000,000      
Repayments of Debt   $ 150,000,000 $ 50,000,000        
Spire Missouri [Member] | Loans Payable [Member] | SOFR [Member]              
Short-Term Debt [Line Items]              
Debt instrument basis spread on variable rate       0.10%      
Spire Missouri [Member] | Loans Payable [Member] | Adjusted SOFR [Member[              
Short-Term Debt [Line Items]              
Debt instrument basis spread on variable rate       0.90%      
Revolving Credit Facility [Member]              
Short-Term Debt [Line Items]              
Line of credit facility maximum borrowing capacity         $ 1,500,000,000    
Line of credit facility covenant percentage of total capitalization         65.00%    
Long-term line of credit         $ 0    
Revolving Credit Facility [Member] | Maximum [Member]              
Short-Term Debt [Line Items]              
Line of credit facility covenant percentage of total capitalization         70.00%    
Revolving Credit Facility [Member] | Spire [Member]              
Short-Term Debt [Line Items]              
Line of credit facility maximum borrowing capacity         $ 525,000,000    
Revolving Credit Facility [Member] | Spire Missouri [Member]              
Short-Term Debt [Line Items]              
Line of credit facility maximum borrowing capacity         700,000,000    
Revolving Credit Facility [Member] | Spire Alabama Inc [Member]              
Short-Term Debt [Line Items]              
Line of credit facility maximum borrowing capacity         $ 275,000,000