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Note 9 - Pension Plans and Other Postretirement Benefits (Details Textual) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Dec. 26, 2022
Dec. 23, 2021
Dec. 22, 2021
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Sep. 30, 2023
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement       $ 0   $ 0    
Regulatory Asset, Total       1,389,600 $ 1,459,700 1,389,600 $ 1,459,700 $ 1,597,500
Regulatory Liability       582,800 365,600 582,800 365,600 479,700
Pension Plan [Member]                
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement       (0) (0) (0) (8,600)  
Spire Alabama Inc [Member]                
Regulatory Asset, Total       634,000 633,100 634,000 633,100 648,500
Regulatory Liability       38,800 21,500 38,800 21,500 21,200
Spire Alabama Inc [Member] | Pension Plan [Member]                
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement           27,500    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement       (0) (0) (0) (8,600)  
Defined Benefit Plan, Plan Assets, Contributions by Employer           5,400    
Anticipated Contributions to Pension Plans Qualified Trusts           11,000    
Spire Alabama Inc [Member] | Two Plans [Member]                
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement           $ (8,600)    
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, 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]                
Anticipated Contributions to Pension Plans Qualified Trusts           $ 0    
Spire Missouri [Member]                
Regulatory Asset, Total       719,700 784,700 719,700 784,700 910,700
Regulatory Liability       $ 475,300 $ 276,900 475,300 $ 276,900 $ 389,400
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 $ 29,900 32,400 29,000          
Spire Missouri [Member] | Pension and Other Postretirement Plans Costs [Member] | Missouri West [Member]                
Regulatory Liabilities For Which No Return On Investment During Recovery Period Is Provided 800 $ 1,100 $ 3,300          
Remaining Recovery Period of Regulatory Liabilities for which No Return on Investment During Recovery Period is Provided (Year)   8 years 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 6,900 $ 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           16,600    
Spire Missouri [Member] | Pension Plan [Member] | Qualified Plan [Member]                
Defined Benefit Plan, Plan Assets, Contributions by Employer           5,000    
Spire Missouri [Member] | Pension Plan [Member] | Nonqualified Plan [Member]                
Defined Benefit Plan, Plan Assets, Contributions by Employer           0    
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member]                
Anticipated Contributions to Pension Plans Qualified Trusts           $ 0    
Postretirement Benefit Plans, Recovery in Rates Base Allowance   $ 8,600            
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri West [Member]                
Regulatory Liability 100              
Spire Missouri [Member] | Other Postretirement Benefits Plan [Member] | Missouri East [Member]                
Regulatory Liability $ 900