Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 456,284	$ 399,361
Research and other credits	132,994	128,015
Operating lease liabilities	35,672	36,907
Liability related to the sale of future royalties	32,737	0
Stock-based compensation	32,517	30,875
Other	11,688	13,568
Deferred tax assets before valuation allowance	701,892	608,726
Valuation allowance for deferred tax assets	(670,103)	(575,087)
Total deferred tax assets	31,789	33,639
Deferred tax liabilities:
Operating lease right-of-use assets	(29,707)	(31,718)
Other	(1,856)	(1,725)
Total deferred tax liabilities	(31,563)	(33,443)
Net deferred tax assets	$ 226	$ 196