REVENUE - Revenue by Major Client (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Product Information [Line Items]
Revenue	$ 325,702	$ 227,375	$ 639,201	$ 492,966
Percent of total revenue	100.00%	100.00%	100.00%	100.00%
Total revenue \| Client A \| Customer concentration risk
Product Information [Line Items]
Percent of total revenue	13.00%

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.

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+ Details