SEGMENT AND GEOGRAPHIC DISCLOSURES - Geographic Breakdown of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	[1]	Sep. 30, 2019	[1]
Revenues from External Customers [Line Items]
Revenues	$ 219,093	$ 290,079	$ 645,236		$ 875,507
United States
Revenues from External Customers [Line Items]
Revenues	147,986	204,659	463,445		641,330
Canada
Revenues from External Customers [Line Items]
Revenues	32,014	37,412	67,658		92,063
Europe
Revenues from External Customers [Line Items]
Revenues	24,673	32,180	76,315		95,558
Other foreign countries
Revenues from External Customers [Line Items]
Revenues	$ 14,420	$ 15,828	$ 37,818		$ 46,556

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