SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 123,600	$ 177,900
Less: accumulated depreciation	(93,432)	(146,814)
Property and equipment	30,215	31,122
Building and leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,700	7,800
Equipment and furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	55,900	72,900
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 59,000	$ 97,200