REVENUE - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 46,041	$ 95,342	$ 96,506	$ 216,786
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue	40,712	87,447	85,963	195,455
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue	3,393	4,055	6,594	9,646
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 1,936	$ 3,840	$ 3,949	$ 11,685