REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of revenue [Line Items]
Revenue	$ 218,556	$ 250,685	$ 415,994	$ 491,168
Revenue recognized that was included in contract liability at the beginning of the period	5,400		11,300
Service revenue
Schedule of revenue [Line Items]
Revenue	208,861	238,107	397,627	467,946
Reimbursable expenses
Schedule of revenue [Line Items]
Revenue	9,008	11,891	17,155	21,920
Non-controlling interests
Schedule of revenue [Line Items]
Revenue	$ 687	$ 687	$ 1,212	$ 1,302