INCOME TAXES - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Taxes [Line Items]
Effective income tax rate expense, percent	15.30%	18.90%	15.80%	15.00%
Income tax expense (benefit)	$ 6,669,000	$ 5,325,000	$ 21,054,000	$ 6,960,000
Deferred tax liability due to expiration of statute of limitations for 2015 federal tax return	1,900,000		1,900,000
Long-term asset	89,957,000		89,957,000		$ 64,526,000
Korean Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes					(36,900,000)
U.S.Federal Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes					16,200,000
KNTS [Member]
Income Taxes [Line Items]
Income tax withholding and interest			13,200,000
Reduction of Korean Withholding Taxes [Member]
Income Taxes [Line Items]
Reduction of deferred tax asset for foreign tax credits and R&D			20,700,000		20,700,000
Allocation of Withholding to Non-Korean Patents [Member]
Income Taxes [Line Items]
Long-term asset	23,500,000		23,500,000		$ 13,600,000
ASU No. 2016-09 [Member]
Income Taxes [Line Items]
Income tax expense (benefit)	$ 693,000	$ 29,000	$ 3,500,000	$ 1,800,000