REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 140,688	$ 141,493	$ 196,535	$ 169,935	$ 217,633	$ 204,575	$ 218,556	$ 197,438	$ 648,651	$ 838,202	$ 942,213
Revenue recognized that was included in the contract liability at the beginning of the period									9,800	20,600
Deferred revenue	5,183				10,108				5,183	10,108
Other current liabilities	14,724				7,030				14,724	7,030
Retained earnings	$ 272,026				$ 590,655				272,026	590,655
Service revenue
Disaggregation of Revenue [Line Items]
Revenue									621,866	805,480	899,561
Reimbursable expenses
Disaggregation of Revenue [Line Items]
Revenue									24,172	30,039	39,912
Non-controlling interests
Disaggregation of Revenue [Line Items]
Revenue									$ 2,613	$ 2,683	$ 2,740
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Deferred revenue												$ 11,200
Unbilled accounts receivable												1,100
Other current liabilities												300
Retained earnings												$ (10,400)