OTHER BALANCE SHEET AMOUNTS - Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 96,251		$ 96,251		$ 86,378
Less: accumulated depreciation and amortization	(62,359)		(62,359)		(49,852)
Total property and equipment, net	33,892		33,892		36,526
Depreciation expense	5,400	$ 3,000	14,300	$ 8,700
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	64,342		$ 64,342		57,482
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			1 year
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	6,898		$ 6,898		6,096
Furniture and fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			3 years
Furniture and fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			7 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 25,011		$ 25,011		$ 22,800
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			1 year
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			7 years