SEGMENT AND GEOGRAPHIC DISCLOSURES - Geographic Breakdown of Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from External Customers [Line Items]
Revenues	[1]	$ 217,410	$ 219,093	$ 650,901	$ 645,236
United States
Revenues from External Customers [Line Items]
Revenues	[1]	143,636	147,986	447,932	463,445
Canada
Revenues from External Customers [Line Items]
Revenues	[1]	30,465	32,014	81,110	67,658
Europe
Revenues from External Customers [Line Items]
Revenues	[1]	27,122	24,673	82,098	76,315
Other foreign countries
Revenues from External Customers [Line Items]
Revenues	[1]	$ 16,187	$ 14,420	$ 39,761	$ 37,818

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