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Incentive Plans (Narrative) (Details)
 1 Months Ended 3 Months Ended 9 Months Ended
Aug. 30, 2013
USD ($)
shares
Feb. 28, 2023
peer_company
$ / shares
Sep. 30, 2023
USD ($)
$ / shares
shares
Mar. 31, 2023
USD ($)
shares
Sep. 30, 2023
USD ($)
$ / shares
shares
Sep. 30, 2022
USD ($)
May 04, 2023
shares
Dec. 31, 2022
$ / shares
shares
Aug. 08, 2013
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based compensation | $         $ 15,800,000 $ 11,700,000      
Total income tax benefits realized from tax deductions related to stock option exercises under share-based payment arrangements | $         $ 400,000 $ 1,100,000      
MUSA 2013 Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Number of shares authorized for incentive plan (no more than) (in shares) 5,500,000                
Number of shares per employee (no more than) (in shares) 1,000,000                
Award expiration period (in years)         7 years        
Shares granted, fair value (in dollars per share) | $ / shares   $ 88.53              
MUSA 2013 Plan | PSUs                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Maximum amount payable | $ $ 5,000,000                
MUSA 2013 Plan | PSUs | Employees                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares         $ 292.52        
Granted (in shares)         62,275        
Vested (in shares)         72,799        
Outstanding (in shares)     95,477   95,477     106,001  
Weighted average grant date fair value (in dollars per share) | $ / shares     $ 218.97   $ 218.97     $ 160.03  
MUSA 2013 Plan | PSUs | ROACE                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)   3 years              
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares   $ 263.48              
MUSA 2013 Plan | PSUs | TSR                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)   3 years              
Number of companies in total shareholder return peer comparison group | peer_company   17              
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares   $ 330.18              
MUSA 2013 Plan | RSUs | Employees                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)         3 years        
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares         $ 256.84        
Granted (in shares)         34,600        
Vested (in shares)         60,735        
Outstanding (in shares)     120,589   120,589     149,366  
Weighted average grant date fair value (in dollars per share) | $ / shares     $ 198.08   $ 198.08     $ 125.51  
2013 Directors Plan | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Number of shares authorized for incentive plan (no more than) (in shares)                 500,000
2013 Directors Plan | RSUs | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)         1 year        
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares         $ 258.35        
Granted (in shares)         6,584        
Vested (in shares)         9,880        
Outstanding (in shares)     23,627   23,627     26,923  
Weighted average grant date fair value (in dollars per share) | $ / shares     $ 177.98   $ 177.98     $ 132.38  
2013 Directors Plan | RSUs | Prior to 2023 | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)         3 years        
2013 Directors Plan | RSUs | In 2023 | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Award vesting period (in years)         1 year        
2013 Directors Plan | DSUs | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share of common stock right     1   1        
Granted (in shares)     1 421          
Vested (in shares)     1            
Share-based compensation | $       $ 100,000          
Outstanding (in shares)     422   422        
Weighted average grant date fair value (in dollars per share) | $ / shares     $ 258.05   $ 258.05        
MUSA 2023 Plan                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Number of shares authorized for incentive plan (no more than) (in shares)             1,725,000    
MUSA 2023 Plan | DSUs | Non-employee directors                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share of common stock right     1   1        
Granted (in shares)     692            
Share-based compensation | $     $ 200,000            
Outstanding (in shares)     692   692        
Weighted average grant date fair value (in dollars per share) | $ / shares     $ 325.32   $ 325.32