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Note 8 - Pension Plans and Other Postretirement Benefits (Details Textual) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Dec. 23, 2021
Dec. 22, 2021
Jun. 30, 2022
Mar. 31, 2022
Jun. 30, 2021
Mar. 31, 2021
Jun. 30, 2022
Feb. 28, 2022
Sep. 30, 2021
Sep. 30, 2020
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement     $ 0 $ 0 $ 0   $ 0      
Regulatory Asset, Total     1,357,600   1,184,500   1,357,600   $ 1,300,000  
Postretirement Benefit Plans, Recovery in Rates Base Allowance $ 8,600                  
Spire Missouri [Member]                    
Regulatory Asset, Total     754,500   620,400   754,500   $ 759,400  
Spire Missouri [Member] | Pension Costs [Member] | Missouri West [Member]                    
Regulatory Asset, Total 4,400 $ 5,500                
Spire Missouri [Member] | Pension Costs [Member] | Missouri East [Member]                    
Regulatory Asset, Total 32,400 29,000                
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 $ 3,300                
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 $ 21,600                
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     47,400 21,600 36,800   47,400      
Defined Benefit Plan, Benefit Obligation, Payment for Settlement     $ 17,300 $ 6,800 $ 9,100   $ 17,300      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate     4.55% 3.60%     4.55% 3.60% 3.00%  
Anticipated Contributions to Pension Plans Qualified Trusts             $ 8,700      
Spire Missouri [Member] | Pension Plan [Member] | Qualified Plan [Member]                    
Defined Benefit Plan, Plan Assets, Contributions by Employer             28,900      
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         3.15%         2.85%
Spire Missouri [Member] | Pension Plan Two [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate         3.10%         2.75%
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]                    
Defined Benefit Plan, Plan Assets, Contributions by Employer             0      
Anticipated Contributions to Pension Plans Qualified Trusts             0      
Spire Alabama Inc [Member]                    
Regulatory Asset, Total     $ 567,000   $ 525,300   567,000   $ 502,100  
Spire Alabama Inc [Member] | Pension Plan [Member]                    
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement     3,600 $ 17,400 7,600   3,600      
Defined Benefit Plan, Benefit Obligation, Payment for Settlement     $ 1,100 $ 4,900 $ 2,100 $ 3,800 $ 1,100      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate     4.55% 3.60% 3.25% 3.20% 4.55% 3.60%   2.95%
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)         11 years 8 months 12 days 11 years 4 months 24 days        
Defined Benefit Plan, Plan Assets, Contributions by Employer             $ 8,500      
Anticipated Contributions to Pension Plans Qualified Trusts             $ 5,900      
Spire Alabama Inc [Member] | Pension Plan One [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate                 3.00%  
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)     13 years 12 years 3 months 18 days     13 years      
Spire Alabama Inc [Member] | Pension Plan Two [Member]                    
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate                 3.10%  
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)     11 years 1 month 6 days 12 years 7 months 6 days     11 years 1 month 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