SEGMENT INFORMATION (Reconciliation of Segment Operating Profit to Consolidated Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment operating profit:	$ 84,745	$ 80,565	$ 72,882	$ 64,658	$ 78,270	$ 64,560	$ 54,237	$ 48,697	$ 302,850	$ 245,764	$ 172,946
Stock-based compensation expense									72,036	59,188	52,407
Amortization of purchased intangibles									9,914	8,511	7,562
Interest and other income, net	1,950	2,509	1,190	3,076	1,080	1,941	1,052	(551)	8,725	3,522	4,601
Foreign exchange (loss)/gain	(1,898)	(3,105)	(3,562)	(3,484)	(582)	(514)	1,830	(247)	(12,049)	487	(3,242)
Income before provision for income taxes	$ 84,797	$ 79,969	$ 70,510	$ 64,250	$ 78,768	$ 65,987	$ 57,119	$ 47,899	299,526	249,773	174,305
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment operating profit:									425,967	349,099	275,326
Unallocated Amounts
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Stock-based compensation expense									(72,036)	(59,188)	(52,407)
Amortization of purchased intangibles									(9,914)	(8,101)	(7,562)
Other acquisition-related expenses									(3,774)	(916)	(1,500)
Other unallocated costs									$ (37,393)	$ (35,130)	$ (40,911)