Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 300,693	$ 364,864
Research and other credits	116,955	94,103
Stock-based compensation	21,518	14,552
Capitalized research expenses	8,072	5,497
Reserves and accruals	8,066	5,659
Deferred revenue	4,467	3,796
Property, plant and equipment	2,124	6,848
Other	(1,270)	1,156
Deferred tax assets before valuation allowance	460,625	496,475
Valuation allowance for deferred tax assets	(460,455)	(496,191)
Total deferred tax assets	170	284
Total deferred tax liabilities	0	0
Net deferred tax assets	$ 170	$ 284