SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized website and internal-use software costs		$ 20,400	$ 19,200	$ 14,700
Amortization expense related to website and internal-use software		16,700	12,300	8,400
Research and development cost		171,200	133,100	104,700
Cumulative effect adjustment upon adoption of ASU 2016-09	[1]			(169)
Advertising expense		50,300	46,900	30,900
Employer contributions		$ 4,800	$ 3,800	2,900
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment upon adoption of ASU 2016-09	[1]			(1,332)
Accounting Standards Update 2016-09 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment upon adoption of ASU 2016-09				$ 1,100

[1] Adopted on a modified retrospective basis; refer to significant accounting policies in Note 2 for details regarding this adoption.