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Note 9 - Pension Plans and Other Postretirement Benefits (Details Textual) - USD ($)
3 Months Ended 6 Months Ended 9 Months Ended
Dec. 23, 2021
Dec. 22, 2021
Mar. 31, 2023
Dec. 31, 2022
Mar. 31, 2022
Mar. 31, 2023
Mar. 31, 2022
Jun. 30, 2022
Sep. 30, 2022
Sep. 30, 2021
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement       $ 0 $ 0          
Regulatory Asset, Total     $ 1,459,700,000   1,270,800,000 $ 1,459,700,000 $ 1,270,800,000   $ 1,467,800,000  
Pension Plan [Member]                    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement     (0)   (11,700,000) (8,600,000) (11,700,000)      
Spire Alabama Inc [Member]                    
Regulatory Asset, Total     633,100,000   547,700,000 633,100,000 547,700,000   $ 595,100,000  
Spire Alabama Inc [Member] | Pension Plan [Member]                    
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement       27,500,000     17,400,000      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement     (0) $ (8,600,000) $ (4,900,000) (8,600,000) $ (4,900,000)      
Defined Benefit Plan, Plan Assets, Contributions by Employer           13,000,000.0        
Anticipated Contributions to Pension Plans Qualified Trusts           $ 21,300,000        
Spire Alabama Inc [Member] | Pension Plan One [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate         3.60%   3.60%     3.00%
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)       13 years 4 months 24 days   12 years 3 months 18 days        
Spire Alabama Inc [Member] | Pension Plan Two [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate         3.10%   3.10%   5.65% 3.00%
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)       12 years 4 months 24 days   12 years 7 months 6 days        
Spire Alabama Inc [Member] | Other Postretirement Benefits Plan [Member]                    
Defined Benefit Plan, Plan Assets, Contributions by Employer               $ 0    
Anticipated Contributions to Pension Plans Qualified Trusts               $ 0    
Spire Missouri [Member]                    
Regulatory Asset, Total     784,700,000   $ 685,400,000 $ 784,700,000 $ 685,400,000   $ 835,700,000  
Spire Missouri [Member] | Pension Costs [Member] | Missouri West [Member]                    
Regulatory Asset, Total $ 4,400,000 $ 5,500,000                
Spire Missouri [Member] | Pension Costs [Member] | Missouri East [Member]                    
Regulatory Asset, Total 32,400,000 29,000,000.0                
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri West [Member]                    
Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided $ 1,100,000 $ 3,300,000                
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)   8 years                
Additional Remaining Recovery Period of Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided (Year) 8 years                  
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri East [Member]                    
Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided $ 11,000,000.0 $ 21,600,000                
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided (Year)   8 years                
Additional Remaining Recovery Period of Regulatory Assets for Which No Return on Investment During Recovery Period Is Provided (Year) 8 years                  
Spire Missouri [Member] | Pension Plan [Member]                    
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement             21,600,000      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement     $ (0)   $ (6,800,000) (0) $ (6,800,000)      
Anticipated Contributions to Pension Plans Qualified Trusts       $ 800,000            
Spire Missouri [Member] | Pension Plan [Member] | Qualified Plan [Member]                    
Defined Benefit Plan, Plan Assets, Contributions by Employer           $ 23.2        
Spire Missouri [Member] | Pension Plan [Member] | Nonqualified Plan [Member]                    
Defined Benefit Plan, Plan Assets, Contributions by Employer       $ 0            
Spire Missouri [Member] | Pension Plan One [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate       5.60% 3.60%   3.60%   5.70% 3.00%
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]                    
Postretirement Benefit Plans, Recovery in Rates Base Allowance $ 8,600,000