REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of revenue [Line Items]
Revenue	$ 197,438	$ 240,483
Revenue recognized that was included in contract liability at the beginning of the period	5,900
Service revenue
Schedule of revenue [Line Items]
Revenue	188,766	229,839
Reimbursable expenses
Schedule of revenue [Line Items]
Revenue	8,147	10,029
Non-controlling interests
Schedule of revenue [Line Items]
Revenue	$ 525	$ 615