SEGMENT INFORMATION (Major Customers) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Revenue, Major 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
Accounts Receivable | Customer Concentration Risk | Billed Revenues
Revenue, Major Customer [Line Items]
Number of customers									0	0
Accounts Receivable | Customer Concentration Risk | Unbilled Revenues
Revenue, Major Customer [Line Items]
Number of customers									1	2
Concentration percentage									13.00%	22.20%
Operating Segments
Revenue, Major Customer [Line Items]
Revenues									$ 1,452,287	$ 1,160,815	914,966
Operating Segments | Europe
Revenue, Major Customer [Line Items]
Revenues									$ 593,167	$ 474,988	$ 400,460
Operating Segments | Sales Revenue, Segment | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Number of customers									0
Operating Segments | Sales Revenue, Segment | Customer Concentration Risk | Europe | UBS AG
Revenue, Major Customer [Line Items]
Number of customers										1	1
Revenues										$ 138,124	$ 130,605