SEGMENT INFORMATION (Revenues by Service Offering) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Revenues	$ 399,297	$ 377,523	$ 348,977	$ 324,651	$ 313,525	$ 298,293	$ 283,832	$ 264,482	$ 1,450,448	$ 1,160,132	$ 914,128
Software development
Revenue from External Customer [Line Items]
Revenues									1,060,286	841,916	644,732
Application testing services
Revenue from External Customer [Line Items]
Revenues									276,270	223,010	174,259
Application maintenance and support
Revenue from External Customer [Line Items]
Revenues									92,630	74,475	70,551
Licensing
Revenue from External Customer [Line Items]
Revenues									3,529	3,141	3,764
Infrastructure services
Revenue from External Customer [Line Items]
Revenues									595	5,069	11,311
Reimbursable expenses and other revenues
Revenue from External Customer [Line Items]
Revenues									$ 17,138	$ 12,521	$ 9,511