SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 129,100	$ 177,900
Less: accumulated depreciation	(101,128)	(146,814)
Property and equipment	27,998	31,122
Building and leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,800	7,800
Equipment and furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	56,900	72,900
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 63,400	$ 97,200