REVENUE - Disaggregation of 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
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
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue									332,084	579,929	719,739
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue									17,178	44,550	88,424
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue									$ 16,285	$ 24,172	$ 30,039