REVENUE RECOGNITION - Summary of Impacts of Adopting Topic 606 on Consolidated Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	[1]	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Other assets (current and non-current)	$ 70,892										$ 70,892
Deferred income taxes	24,377				$ 27,022						24,377	$ 27,022
TOTAL ASSETS	933,424				779,956						933,424	779,956
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	122,567				38,883						122,567	38,883
Retained earnings	129,552				99,126						129,552	99,126
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	933,424				779,956						933,424	779,956
REVENUE	70,143	$ 77,550	$ 56,149	$ 43,572	115,867	[1],[2]	$ 61,683	$ 102,513		$ 55,566	247,414	335,629	$ 198,886
Gross margin											193,873	280,931	172,598
OPERATING INCOME											56,735	146,247	68,413
INCOME BEFORE INCOME TAXES											64,311	149,537	68,598
INCOME TAX EXPENSE											(5,471)	(45,652)	(20,528)
NET INCOME	19,249	$ 22,818	$ 10,814	$ 5,959	$ 32,813	[1],[2]	$ 13,520	$ 47,187		$ 10,365	58,840	103,885	48,070	$ 48,070
Amortization of deferred revenue and recognition of unbilled receivables											(68,905)	(11,122)	(7,406)
Deferred income tax expense											(12,814)	24,396	3,094
Other assets (current and non-current)											(59,062)
CASH FLOW FROM OPERATING ACTIVITIES											121,796	$ 133,365	$ 80,338
Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
LIABILITIES AND SHAREHOLDERS’ EQUITY
REVENUE											78,900
Adjustment [Member] | Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
ASSETS
Deferred income taxes	(11,153)										(11,153)
TOTAL ASSETS	(11,153)										(11,153)
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	(99,885)										(99,885)
Retained earnings	88,732										88,732
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(11,153)										(11,153)
REVENUE											78,885
Gross margin											78,885
OPERATING INCOME											78,885
INCOME BEFORE INCOME TAXES											78,885
INCOME TAX EXPENSE											(7,252)
NET INCOME											71,633
Amortization of deferred revenue and recognition of unbilled receivables											(81,991)
Deferred income tax expense											10,358
Balances Without Adoption of Topic 606 [Member] | Revenue from Contracts with Customers - ASU No. 2014-09 [Member]
ASSETS
Other assets (current and non-current)	70,892										70,892
Deferred income taxes	13,224										13,224
TOTAL ASSETS	922,271										922,271
LIABILITIES AND SHAREHOLDERS’ EQUITY
Deferred revenue (current and non-current)	22,682										22,682
Retained earnings	218,284										218,284
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 922,271										922,271
REVENUE											326,299
Gross margin											272,758
OPERATING INCOME											135,620
INCOME BEFORE INCOME TAXES											143,196
INCOME TAX EXPENSE											(12,723)
NET INCOME											130,473
Amortization of deferred revenue and recognition of unbilled receivables											(150,896)
Deferred income tax expense											(2,456)
Other assets (current and non-current)											(59,062)
CASH FLOW FROM OPERATING ACTIVITIES											$ 121,796

[1] Prior to the adoption of ASC 606 on January 1, 2018, the Company recorded as revenue the receipt of the semi-annual license fee payment of $45.0 million from SDC in the second and fourth quarters.

[2] The enactment of the Tax Cuts and Jobs Act in December 2017 resulted in a one-time charge of $11.5 million in the fourth quarter.