SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property and equipment, net of accumulated depreciation and amortization:
Property and equipment, net	$ 70,859	$ 53,292
Computer equipment and capitalized software | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	2 years
Computer equipment and capitalized software | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	3 years
Furniture and other equipment | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	5 years
Furniture and other equipment | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	7 years
Buildings and leasehold improvements | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	5 years
Buildings and leasehold improvements | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful life	25 years
Capitalized internal use software
Property and equipment, net of accumulated depreciation and amortization:
Property and equipment, net	$ 37,600	$ 23,300