NEW ACCOUNTING STANDARDS (TABLES) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Income Statement [Abstract]
Provision for income taxes	$ 653	$ 1,597	$ 2,755	$ 6,988
Net income	6,297	$ 15,794	12,229	$ 35,961
ASSETS.
Prepaids and other current assets	70,956		70,956		$ 74,465	$ 63,668
Deferred tax assets, net, noncurrent	17,477		17,477		16,018	12,012
Liabilities.
Deferred revenue	25,132		25,132		46,413	21,628
Equity [Abstract]
Retained earnings	712,775		712,775		711,682	$ 721,664
Accounting Standards Update 2014-09 [Member]
Income Statement [Abstract]
Revenue	349,853		725,102
Cost of services	274,260		557,630
Provision for income taxes	653		2,755
Net income	6,297		12,229
ASSETS.
Prepaids and other current assets	70,956		70,956
Deferred tax assets, net, noncurrent	17,477		17,477
Liabilities.
Deferred revenue	25,132		25,132
Equity [Abstract]
Retained earnings	712,775		712,775
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Income Statement [Abstract]
Revenue	347,450		710,198
Cost of services	273,462		550,642
Provision for income taxes	203		539
Net income	5,141		6,530
ASSETS.
Prepaids and other current assets	67,148		67,148
Deferred tax assets, net, noncurrent	15,641		15,641
Liabilities.
Deferred revenue	15,251		15,251
Equity [Abstract]
Retained earnings	721,066		721,066
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Income Statement [Abstract]
Revenue	2,403		14,904
Cost of services	798		6,988
Provision for income taxes	450		2,216
Net income	1,156		5,699
ASSETS.
Prepaids and other current assets	3,808		3,808		10,797
Deferred tax assets, net, noncurrent	1,836		1,836		4,006
Liabilities.
Deferred revenue	9,881		9,881		24,785
Equity [Abstract]
Retained earnings	$ (8,291)		$ (8,291)		$ (9,982)