SEGMENT REPORTING - Schedule of Certain Assets by Reportable Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 1,508,356	$ 1,525,298
Goodwill	543,028	527,023
Permits and other intangibles, net	366,497	386,620
Goodwill and Permits and other intangibles, net	909,525	913,643
Environmental Services
Segment Reporting Information [Line Items]
Goodwill	402,025	401,918
Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Goodwill	141,003	125,105
Operating Segments | Environmental Services
Segment Reporting Information [Line Items]
Property, plant and equipment, net	1,050,901	1,068,910
Goodwill	402,025	401,918
Permits and other intangibles, net	217,188	228,237
Goodwill and Permits and other intangibles, net	619,213	630,155
Operating Segments | Safety-Kleen Sustainability Solutions
Segment Reporting Information [Line Items]
Property, plant and equipment, net	363,857	366,160
Goodwill	141,003	125,105
Permits and other intangibles, net	149,309	158,383
Goodwill and Permits and other intangibles, net	290,312	283,488
Corporate, non-Segment
Segment Reporting Information [Line Items]
Property, plant and equipment, net	$ 93,598	$ 90,228