STOCKHOLDERS’ EQUITY / DEFICIT - Stock Option Activity (Details) - Stock Options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Shares
Outstanding, Beginning Balance (in shares)	4,892,208
Granted (in shares)	2,285,000
Forfeited and expired (in shares)	(970,170)
Exercised (in shares)	(541,454)
Outstanding, Ending Balance (in shares)	5,665,584	4,892,208
Exercisable ending balance (in shares)	2,036,906	1,225,833
Weighted Average Exercise Price
Outstanding, Beginning Balance (in dollars per share)	$ 10.93
Granted (in dollars per share)	27.15
Forfeited and expired (in dollars per share)	14.91
Exercised (in dollars per share)	13.10
Outstanding, Ending Balance (in dollars per share)	17.05	$ 10.93
Exercisable ending balance (in dollars per share)	$ 12.55	$ 9.73
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Remaining Contractual Term, Outstanding (in years)	8 years 9 months	8 years 6 months 25 days
Remaining Contractual Term, Exercisable (in years)	8 years 4 months 2 days	7 years 11 months 16 days
Outstanding, Aggregate Intrinsic Value	$ 31,542	$ 8,353
Exercisable, Aggregate Intrinsic Value	$ 15,582	$ 3,395