SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 159,100	$ 177,900
Less: accumulated depreciation	(130,877)	(146,814)
Property and equipment	28,213	31,122
Building and leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,800	7,800
Equipment and furniture
Property, Plant and Equipment [Line Items]
Property and equipment, gross	63,000	72,900
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 88,300	$ 97,200