SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Balances Related to Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 177,900	$ 175,600
Less accumulated depreciation	(146,814)	(138,650)
Property and equipment, net	31,122	36,999
Building and Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,800	6,900
Equipment and Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	72,900	71,900
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 97,200	$ 96,800