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STOCK-BASED COMPENSATION - Stock Option Activity (Details) - Stock options
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Shares  
Outstanding at beginning of period (in shares) | shares 1,932,259
Granted (in shares) | shares 411,956
Exercised (in shares) | shares (320,408)
Forfeited (in shares) | shares (33,336)
Outstanding at end of period (in shares) | shares 1,990,471
Exercisable at end of period (in shares) | shares 787,504
Weighted-Average Exercise Price  
Outstanding at beginning of period (in dollars per share) | $ / shares $ 17.12
Granted (in dollars per share) | $ / shares 37.57
Exercised (in dollars per share) | $ / shares 18.14
Forfeited (in dollars per share) | $ / shares 21.48
Outstanding at end of period (in dollars per share) | $ / shares 21.09
Exercisable at end of period (in dollars per share) | $ / shares $ 13.85
Weighted Average Remaining Contractual Term  
Outstanding at end of period 7 years 2 months 12 days
Exercisable at end of period 5 years 6 months
Aggregate Intrinsic Value  
Outstanding at end of period (in dollars) | $ $ 47,371,054
Exercisable at end of period (in dollars) | $ $ 24,391,756