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Note 8 - Pension Plans and Other Postretirement Benefits (Details Textual) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 23, 2021
Dec. 22, 2021
Dec. 31, 2022
Dec. 31, 2021
Jun. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement     $ 0 $ 0      
Regulatory Asset, Total     1,489,800 1,355,600     $ 1,467,800
Spire Alabama Inc [Member]              
Regulatory Asset, Total     627,700 516,300     $ 595,100
Spire Alabama Inc [Member] | Pension Plan [Member]              
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement     27,500        
Defined Benefit Plan, Benefit Obligation, Payment for Settlement     8,600        
Defined Benefit Plan, Plan Assets, Contributions by Employer     13,000        
Anticipated Contributions to Pension Plans Qualified Trusts     $ 27,800        
Spire Alabama Inc [Member] | Pension Plan One [Member]              
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)     13 years 4 months 24 days        
Spire Alabama Inc [Member] | Pension Plan Two [Member]              
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate             5.65%
Defined Benefit Plan, Benefit Obligation, Settlement Losses Amortized Over Remaining Actuarial Life (Year)     12 years 4 months 24 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     $ 825,900 $ 797,400     $ 835,700
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]              
Anticipated Contributions to Pension Plans Qualified Trusts     800        
Spire Missouri [Member] | Pension Plan [Member] | Qualified Plan [Member]              
Defined Benefit Plan, Plan Assets, Contributions by Employer     16,700        
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%       5.70%
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]              
Postretirement Benefit Plans, Recovery in Rates Base Allowance $ 8,600            
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Forecast [Member]              
Anticipated Contributions to Pension Plans Qualified Trusts           $ 0