ACQUISITIONS (Fair Values of Net Assets Acquired) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 119,531	$ 109,289	$ 115,930
2015 Acquisitions | Final Valuation
Business Acquisition [Line Items]
Cash and cash equivalents		3,044
Accounts receivable and other current assets		13,854
Property and equipment and other long-term assets		1,830
Deferred tax assets		5,722
Acquired intangible assets		25,500
Goodwill		48,248
Total assets acquired		98,198
Accounts payable and accrued expenses		3,631
Bank loans and other long-term liabilities		295
Deferred revenue		1,099
Due to employees		2,938
Deferred tax liabilities		7,974
Total liabilities assumed		15,937
Net assets acquired		82,261
2015 Acquisitions | Final Valuation | NavigationArts
Business Acquisition [Line Items]
Cash and cash equivalents		1,317
Accounts receivable and other current assets		3,920
Property and equipment and other long-term assets		230
Deferred tax assets		0
Acquired intangible assets		2,800
Goodwill		23,794
Total assets acquired		32,061
Accounts payable and accrued expenses		871
Bank loans and other long-term liabilities		0
Deferred revenue		50
Due to employees		596
Deferred tax liabilities		0
Total liabilities assumed		1,517
Net assets acquired		30,544
2015 Acquisitions | Final Valuation | AGS
Business Acquisition [Line Items]
Cash and cash equivalents		1,727
Accounts receivable and other current assets		9,934
Property and equipment and other long-term assets		1,600
Deferred tax assets		5,722
Acquired intangible assets		22,700
Goodwill		24,454
Total assets acquired		66,137
Accounts payable and accrued expenses		2,760
Bank loans and other long-term liabilities		295
Deferred revenue		1,049
Due to employees		2,342
Deferred tax liabilities		7,974
Total liabilities assumed		14,420
Net assets acquired		$ 51,717