NEW ACCOUNTING STANDARDS - CUMULATIVE EFFECT OF CHANGES DUE TO ADOPTION OF ASU 2014-09 (TABLES) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS.
Prepaids and other current assets	$ 70,956	$ 74,465	$ 63,668
Deferred tax assets, net, noncurrent	17,477	16,018	12,012
LIABILITIES.
Deferred revenue	25,132	46,413	21,628
Retained earnings	712,775	711,682	$ 721,664
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
ASSETS.
Prepaids and other current assets	3,808	10,797
Deferred tax assets, net, noncurrent	1,836	4,006
LIABILITIES.
Deferred revenue	9,881	24,785
Retained earnings	$ (8,291)	$ (9,982)