v3.25.2
Financing - Schedule of Short-term Borrowings (Details) - USD ($)
$ in Millions
9 Months Ended
Jun. 30, 2025
Sep. 30, 2024
Jun. 30, 2024
Short-Term Debt [Line Items]      
Highest borrowings outstanding $ 1,223.0    
Lowest borrowings outstanding 896.0    
Weighted average borrowings $ 1,058.7    
Weighted average interest rate 4.50%    
Borrowings outstanding $ 1,009.5 $ 947.0 $ 771.0
Weighted average interest rate 4.30% 5.20% 5.60%
Term Loan [Member] | Spire Missouri [Member]      
Short-Term Debt [Line Items]      
Highest borrowings outstanding $ 0.0    
Lowest borrowings outstanding 0.0    
Weighted average borrowings $ 0.0    
Weighted average interest rate 0.00%    
Borrowings outstanding $ 0.0 $ 0.0 $ 0.0
Weighted average interest rate 0.00% 0.00% 0.00%
Spire Note [Member] | Spire Missouri [Member]      
Short-Term Debt [Line Items]      
Highest borrowings outstanding $ 615.0    
Lowest borrowings outstanding 299.5    
Weighted average borrowings $ 485.9    
Weighted average interest rate 4.80%    
Borrowings outstanding $ 398.7 $ 495.3 $ 418.3
Weighted average interest rate 4.70% 5.20% 5.60%
Spire Note [Member] | Spire Alabama Inc [Member]      
Short-Term Debt [Line Items]      
Highest borrowings outstanding $ 73.2    
Lowest borrowings outstanding 1.2    
Weighted average borrowings $ 42.1    
Weighted average interest rate 4.80%    
Borrowings outstanding $ 56.4 $ 48.4 $ 23.0
Weighted average interest rate 4.70% 5.20% 5.60%
Parent Company [Member] | Commercial Paper [Member]      
Short-Term Debt [Line Items]      
Highest borrowings outstanding $ 1,223.0    
Lowest borrowings outstanding 896.0    
Weighted average borrowings $ 1,058.7    
Weighted average interest rate 4.50%    
Borrowings outstanding $ 1,009.5 $ 947.0 $ 771.0
Weighted average interest rate 4.30% 5.20% 5.60%