QUARTERLY SUPPLEMENTAL FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1],[2]	Sep. 30, 2017	Jun. 30, 2017	[1]	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 70,143	$ 77,550	$ 56,149	$ 43,572	$ 115,867		$ 61,683	$ 102,513		$ 55,566	$ 247,414	$ 335,629	$ 198,886
Net income	$ 19,249	$ 22,818	$ 10,814	$ 5,959	$ 32,813		$ 13,520	$ 47,187		$ 10,365	$ 58,840	$ 103,885	$ 48,070	$ 48,070
Net income per common share:
Basic	$ 0.40	$ 0.48	$ 0.23	$ 0.13	$ 0.70		$ 0.28	$ 0.99		$ 0.22	$ 1.24	$ 2.19	$ 1.02	$ 1.02
Diluted	$ 0.40	$ 0.48	$ 0.23	$ 0.13	$ 0.69		$ 0.28	$ 0.99		$ 0.22	$ 1.24	$ 2.18	$ 1.02	$ 1.02

[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.