REVENUE - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 88,795	$ 141,493	$ 305,581	$ 507,963
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue	82,067	125,939	277,522	452,643
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue	3,918	8,341	13,564	38,836
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 2,810	$ 7,213	$ 14,495	$ 16,484