SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Recent Accounting Pronouncements Not Yet Effective) (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]
General and administrative expense		$ 105,673	$ 97,481	$ 80,866
Cumulative effect adjustment upon adoption of ASU 2014-09	[1]			169
Sales and marketing expense		438,643	382,854	301,764
Gain on disposal of a business unit		164,779	0	0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment upon adoption of ASU 2014-09	[1]			1,332
Retained Earnings | Pro Forma | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
General and administrative expense		4,000	3,000
Cumulative effect adjustment upon adoption of ASU 2014-09				$ 6,000
Contract Revenue Cost		11,300	10,600
Amortization		10,100	7,700
Sales and marketing expense		(1,200)	$ (2,900)
Gain on disposal of a business unit		$ (1,100)

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