REVENUE RECOGNITION - Summary of Impacts of Adopting Topic 606 on Consolidated Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
ASSETS
Other assets (current and non-current)	$ 60,395		$ 60,395
Deferred income taxes	10,534		10,534		$ 27,022
TOTAL ASSETS	833,249		833,249		779,956
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	97,591		97,591		38,883
Retained earnings	92,346		92,346		99,126
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	833,249		833,249		$ 779,956
REVENUE	56,149	$ 102,513	99,721	$ 158,079
Gross margin	44,514	91,203	80,628	133,782
OPERATING INCOME	10,911	60,519	15,430	72,577
INCOME BEFORE INCOME TAXES	12,665	61,321	18,408	74,031
INCOME TAX EXPENSE	(1,851)	(14,134)	(1,635)	(16,479)
NET INCOME	10,814	$ 47,187	16,773	57,552
Amortization of deferred revenue and recognition of unbilled receivables			(29,266)	(4,873)
Deferred income tax expense			19,312	7,408
Other assets (current and non-current)			(28,633)
CASH FLOW FROM OPERATING ACTIVITIES			49,952	$ 56,629
Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
REVENUE	17,500		42,100
Adjustment [Member] | Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
ASSETS
Other assets (current and non-current)	(2,246)		(2,246)
Deferred income taxes	(10,970)		(10,970)
TOTAL ASSETS	(13,216)		(13,216)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	(65,368)		(65,368)
Retained earnings	52,152		52,152
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(13,216)		(13,216)
REVENUE	17,470		42,123
Gross margin	17,470		42,123
OPERATING INCOME	17,470		42,123
INCOME BEFORE INCOME TAXES	17,470		42,123
INCOME TAX EXPENSE	(3,180)		(7,864)
NET INCOME	14,290		34,259
Amortization of deferred revenue and recognition of unbilled receivables			(47,475)
Deferred income tax expense			10,970
Other assets (current and non-current)			2,246
Balances Without Adoption of Topic 606 [Member] | Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
ASSETS
Other assets (current and non-current)	58,149		58,149
Deferred income taxes	(436)		(436)
TOTAL ASSETS	820,033		820,033
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	32,223		32,223
Retained earnings	144,498		144,498
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	820,033		820,033
REVENUE	73,619		141,844
Gross margin	61,984		122,751
OPERATING INCOME	28,381		57,553
INCOME BEFORE INCOME TAXES	30,135		60,531
INCOME TAX EXPENSE	(5,031)		(9,499)
NET INCOME	$ 25,104		51,032
Amortization of deferred revenue and recognition of unbilled receivables			(76,741)
Deferred income tax expense			30,282
Other assets (current and non-current)			(26,387)
CASH FLOW FROM OPERATING ACTIVITIES			$ 49,952