SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS Prior Period Statements of Income Adjustments, Adoption of ASU 2014-09 (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NET REVENUES		$ 108,249		$ 111,255	$ 107,563	$ 104,688	$ 102,436	$ 101,625	$ 97,571	$ 88,036	$ 431,755	$ 389,668	$ 344,609
COST OF REVENUES											218,091	197,477	171,309
PROVISION FOR INCOME TAXES											32,690	1,054	179
Net income		$ (16,898)	[1]	$ 16,506	$ 13,902	$ 14,099	$ 14,303	$ 12,809	$ 11,407	$ 10,379	$ 27,609	$ 48,898	$ 39,152
Basic		$ (0.57)		$ 0.55	$ 0.47	$ 0.48	$ 0.49	$ 0.44	$ 0.40	$ 0.36	$ 0.93	$ 1.69	$ 1.35
Diluted	[2]	$ (0.57)		$ 0.54	$ 0.46	$ 0.47	$ 0.48	$ 0.43	$ 0.39	$ 0.35	$ 0.90	$ 1.65	$ 1.32
As Reported
NET REVENUES												$ 387,393	$ 343,989
COST OF REVENUES												196,232	170,602
PROVISION FOR INCOME TAXES												1,032	271
Net income												$ 47,890	$ 39,147
Basic												$ 1.66	$ 1.35
Diluted												$ 1.62	$ 1.32
Adjustments for Accounting Standards Update 2014-09 [Member]
NET REVENUES												$ 2,275	$ 620
COST OF REVENUES												1,245	707
PROVISION FOR INCOME TAXES												22	(92)
Net income												$ 1,008	$ 5
Basic												$ 0.03	$ 0.00
Diluted												$ 0.03	$ 0.00

[1] In December 2017 the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (Refer to Note 11, Provision for Income Taxes, in the Notes to Consolidated Financial Statements).

[2] The Company includes the shares underlying performance-based awards in the calculation of diluted earnings per share if the performance conditions have been satisfied as of the end of the reporting period and excludes such shares when the necessary conditions have not been met. The Company has included in the 2017, 2016 and 2015 calculations those shares that were contingently issuable upon the satisfaction of the performance conditions as of the end of the respective periods.