GOODWILL - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Balance at beginning of period	$ 475,025	$ 475,206
Foreign currency translation adjustment	220	(181)
Balance at end of period	475,245	475,025
FMS
Goodwill [Roll Forward]
Balance at beginning of period	243,702	243,606
Foreign currency translation adjustment	103	96
Balance at end of period	243,805	243,702
Accumulated impairment losses	(26,000)	(26,000)
SCS
Goodwill [Roll Forward]
Balance at beginning of period	190,515	190,792
Foreign currency translation adjustment	117	(277)
Balance at end of period	190,632	190,515
Accumulated impairment losses	(19,000)	(19,000)
DTS
Goodwill [Roll Forward]
Balance at beginning of period	40,808	40,808
Foreign currency translation adjustment	0	0
Balance at end of period	$ 40,808	$ 40,808