REVENUE - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 50,465	$ 121,444
Revenue recognized when services are performed or assets are sold
Disaggregation of Revenue [Line Items]
Revenue	45,251	108,008
Revenue related to technology platforms and professional services
Disaggregation of Revenue [Line Items]
Revenue	3,201	5,591
Reimbursable expenses revenue
Disaggregation of Revenue [Line Items]
Revenue	$ 2,013	$ 7,845

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details