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Stockholder's Equity - Nonvested Stock Activity (Details)
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Restricted stock  
Number of shares  
Share grants outstanding as of December 31, 2018 | shares 574,285
Granted (in shares) | shares 159,845
Vested (in shares) | shares (191,707)
Share grants outstanding as of June 30, 2019 | shares 542,423
Weighted average grant date fair value  
Beginning balance, Weighted average grant date fair value (in dollars per share) | $ / shares $ 49.68
Granted (in dollars per share) | $ / shares 109.29
Vested (in dollars per share) | $ / shares 43.71
Ending balance, Weighted average grant date fair value (in dollars per share) | $ / shares $ 68.81
Options  
Number of shares  
Share grants outstanding as of December 31, 2018 | shares 161,807
Granted (in shares) | shares 0
Vested or exercised (in shares) | shares (47,680)
Share grants outstanding as of June 30, 2019 | shares 114,127
Weighted average grant date fair value  
Share grants outstanding as of December 31, 2018 (in dollars per share) | $ / shares $ 38.08
Granted (in dollars per share) | $ / shares 0
Vested or exercised (in dollars per share) | $ / shares 35.55
Share grants outstanding as of June 30, 2019 (in dollars per share) | $ / shares $ 39.59