SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Capitalized software, leasehold improvements and equipment, net:
Property and equipment, net	$ 103,361	$ 70,859
Capitalized software and computer equipment | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	2 years
Capitalized software and computer equipment | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	3 years
Furniture and other equipment | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	5 years
Furniture and other equipment | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	7 years
Leasehold improvements | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	5 years
Leasehold improvements | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life	25 years
Capitalized internal use software
Capitalized software, leasehold improvements and equipment, net:
Property and equipment, net	$ 56,300	$ 37,600