SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Buildings and leasehold improvements | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	3 years
Buildings and leasehold improvements | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	39 years
Computer equipment and capitalized software | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	2 years
Computer equipment and capitalized software | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	3 years
Furniture and other equipment | Minimum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	3 years
Furniture and other equipment | Maximum
Property and equipment, net of accumulated depreciation and amortization:
Estimated useful lives, minimum (in years)	12 years