FAIR VALUE MEASUREMENTS - Acquisition-related Contingent Consideration (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase from revaluation of contingent consideration	$ (5,563)	$ (553)
Recurring basis | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at January 1, 2018	90,000
Increase from revaluation of contingent consideration	5,786
Contractual payments included in accrued liabilities at June 30, 2018	(2,149)
Contractual payments	(2,137)
Balance at June 30, 2018	$ 91,500