PROPERTY, EQUIPMENT AND SOFTWARE, NET - Long-Lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	$ 22,351	$ 0	$ 0
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	413
Leasehold improvements
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	8,419
Office equipment
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	198
Purchased software
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	14
Computer hardware
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	2,842
Capitalized software
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	304
Internally-developed software
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	2,988
Total
Property, Plant and Equipment [Line Items]
Long-lived asset impairment	$ 15,178