INCOME TAXES - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Net operating loss carryforwards	$ 58,624	$ 77,425
Research and development tax credits	24,499	24,440
Other tax credits	0	230
Intangible assets	0	9,270
Deferred revenue	5,886	3,176
Accrued expenses	10,786	6,699
Inventory	5,980	5,010
Stock-based compensation	7,452	14,295
Fixed assets	727	2,879
Other temporary differences	1,556	13
Deferred tax assets, gross	115,510	143,437
Valuation allowance	(89,600)	(141,895)
Total deferred tax assets	25,910	1,542
Liabilities:
Purchased intangible assets	(17,092)	(3,047)
Unremitted foreign income	(3,171)	0
Total deferred tax liabilities	(20,263)	(3,047)
Total net deferred tax assets	5,647
Total net deferred tax assets		(1,505)
Deferred income taxes - noncurrent assets	8,434	1,542
Deferred income taxes - noncurrent liabilities	$ (2,787)	$ (3,047)