GOODWILL AND IMPAIRMENT CHARGES - Goodwill Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Goodwill [Roll Forward]
Goodwill, Gross, beginning balance				$ 358,380
Accumulated Impairment, beginning balance				(267,029)
Goodwill, Net, beginning balance				91,351
Goodwill, Foreign Currency Translation Gain (Loss)				(1,333)
Goodwill, Gross, Impairment charge				0
Accumulated Impairment, Impairment charge				55,837
Goodwill, Net, Impairment charge	$ (55,837)	$ 0		(55,837)	$ (191,788)
Goodwill, Gross, ending balance	357,047			357,047
Accumulated Impairment, ending balance	(322,866)			(322,866)
Goodwill, Net, ending balance	34,181			34,181
IHT
Goodwill [Roll Forward]
Goodwill, Gross, beginning balance				212,928
Accumulated Impairment, beginning balance				(212,928)
Goodwill, Net, beginning balance				0
Goodwill, Net, Impairment charge			$ (191,800)
Goodwill, Gross, ending balance	212,928			212,928
Accumulated Impairment, ending balance	(212,928)			(212,928)
Goodwill, Net, ending balance	0			0
MS
Goodwill [Roll Forward]
Goodwill, Gross, beginning balance				110,721
Accumulated Impairment, beginning balance				(54,101)
Goodwill, Net, beginning balance				56,620
Goodwill, Foreign Currency Translation Gain (Loss)				(783)
Goodwill, Gross, Impairment charge				0
Accumulated Impairment, Impairment charge				55,837
Goodwill, Net, Impairment charge				(55,837)
Goodwill, Gross, ending balance	109,938			109,938
Accumulated Impairment, ending balance	(109,938)			(109,938)
Goodwill, Net, ending balance	0			0
Quest Integrity
Goodwill [Roll Forward]
Goodwill, Gross, beginning balance				34,731
Accumulated Impairment, beginning balance				0
Goodwill, Net, beginning balance				34,731
Goodwill, Foreign Currency Translation Gain (Loss)				(550)
Goodwill, Gross, ending balance	34,181			34,181
Accumulated Impairment, ending balance	0			0
Goodwill, Net, ending balance	$ 34,181			$ 34,181