GOODWILL AND OTHER INTANGIBLE ASSETS - Changes to Goodwill (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Goodwill [Roll Forward]
Balance at January 1, 2021	$ 527,023
Increase from current period acquisition	15,860
Foreign currency translation	145
Balance at September 30, 2021	543,028
Environmental Services
Goodwill [Roll Forward]
Balance at January 1, 2021	401,918
Increase from current period acquisition	0
Foreign currency translation	107
Balance at September 30, 2021	402,025
Safety-Kleen Sustainability Solutions
Goodwill [Roll Forward]
Balance at January 1, 2021	125,105
Increase from current period acquisition	15,860
Foreign currency translation	38
Balance at September 30, 2021	$ 141,003