SEGMENT AND GEOGRAPHIC DISCLOSURES - Geographic Breakdown of Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues from External Customers [Line Items]
Revenues	[1]	$ 238,873	$ 189,304	$ 433,491	$ 426,143
United States
Revenues from External Customers [Line Items]
Revenues	[1]	162,465	141,950	304,296	315,460
Canada
Revenues from External Customers [Line Items]
Revenues	[1]	34,152	14,435	50,647	35,643
Europe
Revenues from External Customers [Line Items]
Revenues	[1]	29,264	23,097	54,975	51,643
Other foreign countries
Revenues from External Customers [Line Items]
Revenues	[1]	$ 12,992	$ 9,822	$ 23,573	$ 23,397

[1] Revenues attributable to individual countries/geographic areas are based on the country of domicile of the legal entity that performs the work.