STOCK OPTIONS, SUMMARY OF OPTION ACTIVITY - (Details) - Stock Options - USD ($)	3 Months Ended
	Mar. 31, 2017
Summary of option activity - Shares
Outstanding at the beginning of year (in shares)	3,193,785
Granted (in shares)	1,114,463
Forfeited (in shares)	(218,089)
Exercised (in shares)	(58,403)
Outstanding at the end of period (in shares)	4,031,756
Vested at the end of period (in shares)	1,831,440
Summary of option activity - Weighted Average Exercise Price
Outstanding at the beginning of year (in dollars per shares)	$ 7.52
Granted (in dollars per shares)	4.34
Forfeited (in dollars per shares)	8.21
Exercised (in dollars per shares)	0.29
Outstanding at the end of period (in dollars per shares)	6.71
Vested at the end of period (in dollars per share)	$ 7.87
Option activity disclosures
Weighted Average Remaining Contractual Term - Outstanding	7 years 10 months 6 days
Weighted Average Remaining Contractual Term - Vested	6 years 3 months 29 days
Aggregate Intrinsic Value - Outstanding (in dollars)	$ 326,568,000
Aggregate Intrinsic Value - Vested (in dollars)	$ 318,837,000
Stock Option
Weighted average grant-date fair value of options granted	$ 3.54
Total fair value of options vested	$ 943,000
Total unrecognized compensation expense, related to non-vested share-based compensation arrangements	$ 10,000,000
Period for unrecognized compensation expense is estimated to be recognized	2 years 8 months 12 days