SEGMENT INFORMATION (Revenues by Client Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 504,931	$ 468,186	$ 445,647	$ 424,148	$ 399,297	$ 377,523	$ 348,977	$ 324,651	$ 1,842,912	$ 1,450,448	$ 1,160,132
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,029,327	783,563	611,392
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									200,918	188,995	177,194
Switzerland
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									144,398	123,281	122,919
Germany
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									80,787	60,158	43,621
Russia
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									71,181	61,222	40,944
Netherlands
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									70,274	51,556	17,521
Canada
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									69,836	57,129	59,189
Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 176,191	$ 124,544	$ 87,352