REVENUE - Disaggregation of 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
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 when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue									579,929	719,739
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue									44,550	88,424
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 24,172	$ 30,039