SEGMENT AND GEOGRAPHIC DISCLOSURES - Geographic Breakdown of Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues from External Customers [Line Items]
Revenues	[1]	$ 290,079	$ 290,856	$ 875,507	$ 937,130
United States
Revenues from External Customers [Line Items]
Revenues	[1]	204,659	207,655	641,330	683,804
Canada
Revenues from External Customers [Line Items]
Revenues	[1]	37,412	32,525	92,063	111,057
Europe
Revenues from External Customers [Line Items]
Revenues	[1]	32,180	32,746	95,558	93,442
Other foreign countries
Revenues from External Customers [Line Items]
Revenues	[1]	$ 15,828	$ 17,930	$ 46,556	$ 48,827

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