NEW ACCOUNTING STANDARDS (TABLES) (Details) - USD ($) $ in Thousands		3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Income Statement [Abstract]
Provision for income taxes		$ 2,102	$ 5,391
Net income		5,932	$ 20,167
ASSETS.
Prepaids and other current assets	$ 71,840	71,840		$ 74,465	$ 63,668
Deferred tax assets, net, noncurrent	16,476	16,476		16,018	12,012
Liabilities.
Deferred revenue	28,675	28,675		46,413	21,628
Equity [Abstract]
Retained earnings	707,257	707,257		711,682	$ 721,664
Accounting Standards Update 2014-09 [Member]
Income Statement [Abstract]
Revenue	375,249
Cost of services	283,370
Provision for income taxes	2,102
Net income	5,932
ASSETS.
Prepaids and other current assets	71,840	71,840
Deferred tax assets, net, noncurrent	16,476	16,476
Liabilities.
Deferred revenue	28,675	28,675
Equity [Abstract]
Retained earnings	707,257	707,257
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Income Statement [Abstract]
Revenue	362,748
Cost of services	277,179
Provision for income taxes	335
Net income	1,389
ASSETS.
Prepaids and other current assets	67,234	67,234
Deferred tax assets, net, noncurrent	14,229	14,229
Liabilities.
Deferred revenue	16,389	16,389
Equity [Abstract]
Retained earnings	712,690	712,690
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Income Statement [Abstract]
Revenue	12,501
Cost of services	6,191
Provision for income taxes	1,767
Net income	4,543
ASSETS.
Prepaids and other current assets	4,606	4,606		10,797
Deferred tax assets, net, noncurrent	2,247	2,247		4,006
Liabilities.
Deferred revenue	12,286	12,286		24,785
Equity [Abstract]
Retained earnings	$ (5,433)	$ (5,433)		$ (9,982)