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Incentive Plans (Schedule of Stock Unit Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2023
Mar. 31, 2023
Sep. 30, 2023
2013 Plan — Employee RSUs | MUSA 2013 Plan | Employees      
Number of units      
Beginning balance (in shares)   149,366 149,366
Granted (in shares)     34,600
Vested (in shares)     (60,735)
Issued (in shares)     (60,735)
Forfeited (in shares)     (2,642)
Ending balance (in shares) 120,589   120,589
Weighted Average Grant Date Fair Value      
Beginning balance (in dollars per share)   $ 125.51 $ 125.51
Granted (in dollars per share)     256.84
Vested (in dollars per share)     80.07
Issued (in dollars per share)     80.07
Forfeited (in dollars per share)     176.19
Ending balance (in dollars per share) $ 198.08   $ 198.08
Total Fair Value      
Total fair value vested     $ 16.0
Total fair value issued     16.0
Total fair value, outstanding $ 41.2   $ 41.2
2013 Plan — Employee RSUs | 2013 Directors Plan | Non-employee directors      
Number of units      
Beginning balance (in shares)   26,923 26,923
Granted (in shares)     6,584
Vested (in shares)     (9,880)
Issued (in shares)     (9,880)
Ending balance (in shares) 23,627   23,627
Weighted Average Grant Date Fair Value      
Beginning balance (in dollars per share)   $ 132.38 $ 132.38
Granted (in dollars per share)     258.35
Vested (in dollars per share)     110.19
Issued (in dollars per share)     110.19
Ending balance (in dollars per share) $ 177.98   $ 177.98
Total Fair Value      
Total fair value vested     $ 2.7
Total fair value issued     2.7
Total fair value, outstanding $ 8.1   $ 8.1
2013 Plan — Employee PSUs | MUSA 2013 Plan | Employees      
Number of units      
Beginning balance (in shares)   106,001 106,001
Granted (in shares)     62,275
Vested (in shares)     (72,799)
Issued (in shares)     (72,799)
Ending balance (in shares) 95,477   95,477
Weighted Average Grant Date Fair Value      
Beginning balance (in dollars per share)   $ 160.03 $ 160.03
Granted (in dollars per share)     292.52
Vested (in dollars per share)     121.83
Issued (in dollars per share)     121.83
Ending balance (in dollars per share) $ 218.97   $ 218.97
Total Fair Value      
Total fair value vested     $ 19.0
Total fair value issued     19.0
Total fair value, outstanding $ 32.6   $ 32.6
Restricted Stock Units (RSUs) And | MUSA 2023 Plan | Non-employee directors      
Number of units      
Beginning balance (in shares)   0 0
Granted (in shares)     692
Vested (in shares)     0
Issued (in shares)     0
Ending balance (in shares) 692   692
Weighted Average Grant Date Fair Value      
Beginning balance (in dollars per share)   $ 0 $ 0
Granted (in dollars per share)     325.32
Vested (in dollars per share)     0
Issued (in dollars per share)     0
Ending balance (in dollars per share) $ 325.32   $ 325.32
Total Fair Value      
Total fair value vested     $ 0.0
Total fair value issued     0.0
Total fair value, outstanding $ 0.2   $ 0.2
DSUs | 2013 Directors Plan | Non-employee directors      
Number of units      
Granted (in shares) 1 421  
Vested (in shares) (1)    
Ending balance (in shares) 422   422
Weighted Average Grant Date Fair Value      
Ending balance (in dollars per share) $ 258.05   $ 258.05
DSUs | MUSA 2023 Plan | Non-employee directors      
Number of units      
Granted (in shares) 692    
Ending balance (in shares) 692   692
Weighted Average Grant Date Fair Value      
Ending balance (in dollars per share) $ 325.32   $ 325.32