SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Capitalized software, leasehold improvements and equipment, net:
Capitalized software, leasehold improvements and equipment, net	$ 118,267	$ 108,842
Capitalized software and computer equipment | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	2 years
Capitalized software and computer equipment | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	3 years
Furniture and other equipment | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	5 years
Furniture and other equipment | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	7 years
Leasehold improvements | Minimum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	5 years
Leasehold improvements | Maximum
Capitalized software, leasehold improvements and equipment, net:
Estimated useful life (in years)	25 years
Capitalized internal use software
Capitalized software, leasehold improvements and equipment, net:
Capitalized software, leasehold improvements and equipment, net	$ 86,400	$ 67,900