INCOME TAXES - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Taxes [Line Items]
Effective income tax rate expense, percent	14.60%	23.00%	8.90%	22.30%
Income tax expense (benefit)	$ 1,851,000	$ 14,134,000	$ 1,635,000	$ 16,479,000
Korean Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes	(37,500,000)		(37,500,000)
U.S.Federal Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes	18,300,000		18,300,000
Reduction of Korean Withholding Taxes [Member]
Income Taxes [Line Items]
Reduction of deferred tax asset for foreign tax credits and R&D			19,200,000
ASU No. 2016-09 [Member]
Income Taxes [Line Items]
Income tax expense (benefit)	$ (448,000)	$ (1,200,000)	$ (1,700,000)	$ (2,400,000)