NEW ACCOUNTING STANDARDS (TABLES) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 364,936		$ 1,090,038
Cost of services	286,925		844,555
Provision for income taxes	1,893	$ 2,071	4,648	$ 9,059
Net income	6,744	$ 15,575	18,973	$ 51,536
ASSETS.
Prepaids and other current assets	86,054		86,054		$ 74,465	$ 63,668
Deferred tax assets, net, noncurrent	22,335		22,335		16,018	12,012
Liabilities.
Deferred revenue	33,555		33,555		46,413	21,628
Equity [Abstract]
Retained earnings	705,218		$ 705,218		711,682	$ 721,664
Revenue, Practical Expedient, Financing Component [true false]			true
Revenue, Remaining Performance Obligation, Optional Exemption, Performance Obligation [true false]			true
Adjustment
ASSETS.
Prepaids and other current assets					10,797
Deferred tax assets, net, noncurrent					4,006
Liabilities.
Deferred revenue					24,785
Equity [Abstract]
Retained earnings					$ (9,982)
ASU 2014-09 | Before Adoption
Income Statement [Abstract]
Revenue	375,053		$ 1,085,251
Cost of services	292,980		843,623
Provision for income taxes	3,030		3,569
Net income	9,668		16,198
ASSETS.
Prepaids and other current assets	76,190		76,190
Deferred tax assets, net, noncurrent	19,432		19,432
Liabilities.
Deferred revenue	13,555		13,555
Equity [Abstract]
Retained earnings	716,433		716,433
ASU 2014-09 | Adjustment
Income Statement [Abstract]
Revenue	(10,117)		4,787
Cost of services	(6,055)		932
Provision for income taxes	(1,137)		1,079
Net income	(2,924)		2,775
ASSETS.
Prepaids and other current assets	9,864		9,864
Deferred tax assets, net, noncurrent	2,903		2,903
Liabilities.
Deferred revenue	20,000		20,000
Equity [Abstract]
Retained earnings	$ (11,215)		$ (11,215)