Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Share Grants Activity (Details)

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Stockholders' Equity - Share Grants Activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Weighted average grant date fair value      
Granted (in dollars per share) $ 110.56 $ 64.11 $ 48.52
Nonvested stock      
Number of Shares      
Share grants outstanding, beginning balance (in shares) 574,285    
Granted (in shares) 184,630    
Vested or exercised (in shares) (224,584)    
Share grants outstanding, ending balance (in shares) 534,331 574,285  
Weighted average grant date fair value      
Share grants outstanding, beginning balance (in dollars per share) $ 49.68    
Granted (in dollars per share) 110.56    
Vested or exercised (in dollars per share) 45.76    
Share grants outstanding, ending balance (in dollars per share) $ 71.01 $ 49.68  
Options      
Number of Shares      
Share grants outstanding, beginning balance (in shares) 161,807    
Granted (in shares) 0    
Vested or exercised (in shares) (63,051)    
Share grants outstanding, ending balance (in shares) 98,756 161,807  
Weighted average grant date fair value      
Share grants outstanding, beginning balance (in dollars per share) $ 38.08    
Granted (in dollars per share) 0    
Vested or exercised (in dollars per share) 35.41    
Share grants outstanding, ending balance (in dollars per share) $ 40.71 $ 38.08