ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Recent Accounting Pronouncement
Accumulated deficit	$ (365,852)		$ (365,852)		$ (360,531)	$ (359,772)
Deferred tax assets	71,967		71,967		68,394	68,126
General and administrative expenses	28,308	$ 25,356	80,467	$ 75,969
Non-service defined benefit plan expense	176	184	528	552
ASU 2014-09 | Adjustments
Recent Accounting Pronouncement
Accumulated deficit	(68)		(68)		(759)	707
Contract liability	1,000		1,000
Deferred tax assets	24		24		$ 268	$ (249)
ASU 2017-07
Recent Accounting Pronouncement
General and administrative expenses	(200)	(200)	(500)	(600)
Non-service defined benefit plan expense	$ 200	$ 200	$ 500	$ 600