INCOME TAXES - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at the federal statutory rate of 21% (35% for 2017 and 2016)	$ 14,705	$ (53,771)	$ 7,862
State income taxes, net of effect of federal tax benefit	4,702	(3,678)	1,063
Stock-based compensation	(25,184)	(32,702)	0
Unbenefited losses	2,227	5,915	2,592
Deferred tax adjustment for enacted changes in tax law and rates	(1,431)	33,002	0
Research credit	(1,169)	(784)	(930)
Other, net	(1,333)	2,912	1,247
Income tax (benefit) provision	$ (7,483)	$ (49,106)	$ 11,834