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Stockholders' Equity - Stock Option Plan and Restricted Stock Activity (Details)
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Stock Options  
Balance at beginning of period (shares) | shares 1,876,332
Granted (shares) | shares 202,994
Options exercised/RSUs vested (shares) | shares (133,800)
Balance at end of period (shares) | shares 1,945,526
Weighted- Average Exercise Price  
Balance at beginning of period (USD per share) | $ / shares $ 53.17
Granted (USD per share) | $ / shares 159.02
Options exercised/RSUs vested (USD per share) | $ / shares 64.73
Balance at end of period (USD per share) | $ / shares $ 63.43
Restricted Stock  
Restricted Stock Awards  
Nonvested at beginning of period (shares) | shares 133,294
Granted (shares) | shares 45,203
Exercised (shares) | shares (53,501)
Nonvested at end of period (shares) | shares 124,996
Weighted- Average Grant Date Fair Value  
Nonvested at beginning of period (USD per share) | $ / shares $ 91.60
Granted (USD per share) | $ / shares 158.29
Options exercised/RSUs vested (USD per share) | $ / shares 80.49
Nonvested at end of period (USD per share) | $ / shares $ 120.48