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Stock Options - Schedule of Stock Options Granted (Details) - $ / shares
3 Months Ended
Mar. 15, 2019
Aug. 24, 2018
Jan. 05, 2018
Mar. 24, 2017
Aug. 27, 2015
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Employee Stock Purchase Plan                  
Shares (in shares)         38,750 160,000 25,000    
Forfeited (in shares)         (15,250) 0 0    
Outstanding (in shares)         23,500 423,700 118,500 263,700 93,500
Strike price (in dollars per share)         $ 42.03 $ 42.01 $ 45.11    
Vesting period (in years)         5 years        
Expiration (in years)         10 years        
Tranche One                  
Employee Stock Purchase Plan                  
Shares (in shares) 80,600 50,200 12,500 46,250          
Forfeited (in shares) 0 0 0 (4,500)          
Outstanding (in shares) 80,600 50,200 12,500 41,750          
Strike price (in dollars per share) $ 42.01 $ 46.00 $ 45.11 $ 44.21          
Vesting period (in years) 3 years 3 years 3 years 3 years          
Expiration (in years) 10 years 10 years 10 years 10 years          
Tranche Two                  
Employee Stock Purchase Plan                  
Shares (in shares) 79,400 99,500 12,500 23,750          
Forfeited (in shares) 0 0 0 0          
Outstanding (in shares) 79,400 99,500 12,500 23,750          
Strike price (in dollars per share) $ 42.01 $ 46.00 $ 45.11 $ 44.21          
Vesting period (in years) 5 years 5 years 5 years 5 years          
Expiration (in years) 10 years 10 years 10 years 10 years