PROPERTY AND EQUIPMENT - NET (Property, Plant and Equipment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 148,125	$ 120,217
Less accumulated depreciation and amortization	(61,706)	(46,601)
Total	86,419	73,616	$ 60,499
Depreciation and amortization expenses	$ 21,000	15,217	$ 11,979
Computer hardware
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	3 years
Property and equipment, gross	$ 62,132	49,599
Building
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	49 years
Property and equipment, gross	$ 34,058	34,012
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	7 years
Property and equipment, gross	$ 18,071	13,178
Leasehold improvements
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	6 years
Property and equipment, gross	$ 13,186	7,944
Office equipment
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	7 years
Property and equipment, gross	$ 10,825	9,416
Purchased computer software
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	3 years
Property and equipment, gross	$ 8,379	4,601
Land improvements
Property, Plant and Equipment [Line Items]
Weighted average useful life (in years)	20 years
Property and equipment, gross	$ 1,474	$ 1,467