SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property, Equipment and Capitalized Software, Net (Details)	12 Months Ended
	Dec. 31, 2020
Office and computer equipment | Minimum
Property, Equipment and Capitalized Software, net
Useful life	3 years
Office and computer equipment | Maximum
Property, Equipment and Capitalized Software, net
Useful life	5 years
Purchased software
Property, Equipment and Capitalized Software, net
Useful life	5 years
Capitalized software | Minimum
Property, Equipment and Capitalized Software, net
Useful life	3 years
Capitalized software | Maximum
Property, Equipment and Capitalized Software, net
Useful life	5 years
Furniture and fixtures
Property, Equipment and Capitalized Software, net
Useful life	7 years
Buildings
Property, Equipment and Capitalized Software, net
Useful life	40 years