REVENUE - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 95,342	$ 196,535	$ 216,786	$ 366,470
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue	87,447	178,949	195,455	326,704
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue	4,055	12,811	9,646	30,495
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 3,840	$ 4,775	$ 11,685	$ 9,271