SHAREHOLDERS' EQUITY AND SHARE-BASED COMPENSATION - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of options
Outstanding at the beginning of the period (in shares)	1,996,509
Granted (in shares)	243,930	145,000	854,000
Exercised (in shares)	(223,060)
Forfeited (in shares)	(271,473)
Outstanding at the end of the period (in shares)	1,745,906	1,996,509
Exercisable at the end of the period (in shares)	1,140,333
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 25.98
Granted (in dollars per share)	33.28	$ 29.17	$ 24.21
Exercised (in dollars per share)	10.64
Forfeited (in dollars per share)	30.12
Outstanding at the end of the period (in dollars per share)	28.20	$ 25.98
Exercisable at the end of the period (in dollars per share)	$ 23.10
Weighted average contractual term (in years)
Weighted average contractual term	4 years 11 months 16 days	5 years 3 months 26 days
Exercisable at the end of the period	3 years 3 months 18 days
Aggregate intrinsic value (in thousands)
Aggregate intrinsic value (in dollars)	$ 10,202	$ 15,942
Exercisable at the end of the period (in dollars)	$ 9,160