INCOME TAXES - Unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Recognized tax benefit	$ 500
Accrued interest and penalties	3,000	$ 2,600
Interest and penalty charges in income tax expense (benefit)	400
Reconciliation of total amounts of unrecognized tax benefits
Balance at beginning of the period	999	1,287	$ 2,760
Gross increases - tax positions in prior period	9
Gross decreases - tax positions in prior period	39	(31)	(916)
Gross increases - tax positions in current period		45	70
Settlement		(103)	(110)
Lapse of statute of limitations		(199)	(517)
Balance at end of the period	$ 1,047	$ 999	$ 1,287