REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 59,966	$ 88,795	$ 95,342	$ 121,444	$ 140,688	$ 141,493	$ 196,535	$ 169,935	$ 365,547	$ 648,651	$ 838,202
Increase in deferred revenue	5,461				5,183				5,461	5,183
Increase in other current liabilities	9,305				14,724				9,305	14,724
Decrease in retained earnings	$ (190,383)				$ (272,026)				(190,383)	(272,026)
Revenue recognized that was included in the contract liability at the beginning of the period									4,800	9,800	20,600
Service revenue
Disaggregation of Revenue [Line Items]
Revenue									347,313	621,866	805,480
Reimbursable expenses
Disaggregation of Revenue [Line Items]
Revenue									16,285	24,172	30,039
Non-controlling interests
Disaggregation of Revenue [Line Items]
Revenue									$ 1,949	$ 2,613	$ 2,683
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Increase in deferred revenue												$ 11,200
Increase in unbilled accounts receivable												1,100
Increase in other current liabilities												300
Decrease in retained earnings												$ 10,400