REVENUE - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 43,243	$ 88,795	$ 139,749	$ 305,581
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue	38,147	82,067	124,111	277,522
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue	3,680	3,918	10,273	13,564
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 1,416	$ 2,810	$ 5,365	$ 14,495