SHARE-BASED PAYMENT - Plan Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted shares
Restricted Shares - Shares
Nonvested at beginning of period (in shares)	1,481,483
Granted (in shares)	385,379
Vested (in shares)	(507,442)
Forfeited (in shares)	(14,897)
Nonvested at end of period (in shares)	1,344,523	1,481,483
Restricted Shares - Weighted Average Grant-date Fair Value
Nonvested at beginning of period (in dollars per share)	$ 20.71
Granted (in dollars per share)	41.15	$ 21.51	$ 21.03
Vested (in dollars per share)	20.08
Forfeited (in dollars per share)	27.67
Nonvested at end of period (in dollars per share)	$ 26.68	$ 20.71
Additional disclosures
Fair value, vested shares (in dollars)	$ 21,200	$ 10,300	$ 9,600
Unrecognized compensation
Unrecognized compensation for outstanding restricted shares/units	$ 18,900
Unrecognized compensation cost, period for recognition	2 years 4 months 24 days
Stock options
Options
Outstanding at beginning of period (in shares)	1,295,375
Granted (in shares)	112,731
Exercised (in shares)	(11,400)
Outstanding at end of period (in shares)	1,396,706	1,295,375
Exercisable at end of period (in shares)	1,061,465
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 16.97
Granted (in dollars per share)	39.82
Exercised (in dollars per share)	46.98
Outstanding at end of period (in dollars per share)	18.85	$ 16.97
Exercisable at end of period (in dollars per share)	$ 16.59
Weighted-Average Remaining Contract Life (Years)
Outstanding at end of period (in years)	6 years 3 months 18 days
Exercisable at end of period (in years)	5 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding at end of period (in dollars)	$ 40,018
Exercisable at end of period (in dollars)	32,812
Intrinsic value of options exercised, (in dollars)	400	$ 200	2,600
Cash received from the exercise of options	0	$ 0	$ 0
Unrecognized compensation
Unrecognized compensation cost for outstanding options	$ 1,800
Unrecognized compensation cost, period for recognition	1 year 9 months 18 days
RSUs
Restricted Shares - Shares
Nonvested at beginning of period (in shares)	1,164,791
Granted (in shares)	335,991		0
Vested (in shares)	(579,589)	0	0
Forfeited (in shares)	(59,875)
Nonvested at end of period (in shares)	861,318	1,164,791
Restricted Shares - Weighted Average Grant-date Fair Value
Nonvested at beginning of period (in dollars per share)	$ 19.61
Granted (in dollars per share)	41.79	$ 23.92
Vested (in dollars per share)	16.07
Forfeited (in dollars per share)	16.07
Nonvested at end of period (in dollars per share)	$ 30.89	$ 19.61
Additional disclosures
Fair value, vested shares (in dollars)	$ 23,100
Unrecognized compensation
Unrecognized compensation for outstanding restricted shares/units	$ 10,000
Unrecognized compensation cost, period for recognition	1 year 7 months 6 days