SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Restructuring Reserve [Roll Forward]
Charged to restructuring costs	$ 1,140	$ 435
Cost Optimization Project [Member] | On-Going Benefit Costs [Member]
Restructuring Reserve [Roll Forward]
Balance at January 1, 2019	556
Charged to restructuring costs	1,104
Payments	(373)
Balance at March 31, 2019	1,287
Cost Optimization Project [Member] | Exit Costs [Member]
Restructuring Cost and Reserve [Line Items]
ASC 842 lease reclassification	$ (300)