PROPERTY AND EQUIPMENT - Components of Property and Equipment, net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 98,232	$ 92,403
Less: accumulated depreciation	(61,597)	(52,044)
Property and equipment, net	36,635	40,359
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	39,216	38,650
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	25,285	23,783
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,517	8,517
Computer software purchased
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,578	6,690
Computer software developed for internal use
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,463	12,596
Fixed assets in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,173	$ 2,167