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Incentive Plans (Summary of Restricted Stock Unit Activity) (Details) - shares
12 Months Ended 28 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2015
MUSA 2013 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Beginning balance 397,015 347,098 0  
Granted 55,450 93,025 352,522  
Vested and issued (66,116) (20,680) (509)  
Forfeited (30,049) (22,428) (4,915)  
Ending balance 356,300 397,015 347,098 356,300
MUSA 2013 Plan [Member] | Performance Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Beginning balance 72,000 0    
Granted 40,400 78,500    
Vested and issued 0 0    
Forfeited (10,006) (6,500)    
Ending balance 102,394 72,000 0 102,394
2013 Stock Plan For Non-Employee Directors [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Granted       63,774
2013 Stock Plan For Non-Employee Directors [Member] | Restricted Stock Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Beginning balance 50,850 28,413 0  
Granted 12,924 22,437 28,413  
Vested and issued 0 0 0  
Forfeited 0 0 0  
Ending balance 63,774 50,850 28,413 63,774