SHAREHOLDERS' EQUITY - Deferred Compensation Arrangement - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangement with Employees and Non-Employees, Share-Based Payments [Line Items]
Fair value of shares withheld for tax withholding obligations	$ 11,620,000	$ 9,432,000	$ 4,871,000
Employees and Non Employee Members of Scientific Advisory Board [Member]
Deferred Compensation Arrangement with Employees and Non-Employees, Share-Based Payments [Line Items]
Shares issued (in shares)	2,456	5,590
Employees [Member]
Deferred Compensation Arrangement with Employees and Non-Employees, Share-Based Payments [Line Items]
Fair value of shares issued	$ 0	$ 165,000
Shares withheld for tax withholding obligations (in shares)		605
Fair value of shares withheld for tax withholding obligations		$ 55,000
Members of Scientific Advisory Board [Member]
Deferred Compensation Arrangement with Employees and Non-Employees, Share-Based Payments [Line Items]
Fair value of shares issued	$ 300,000	$ 300,000