Income taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$ 546,098	$ 439,839
Tax credit carryforwards (federal and state)	246,742	213,810
Capitalized license fees and research and development expenses	14,402	16,295
Deferred revenue	15,644	15,119
Stock-based compensation	51,828	46,111
Lease liabilities	48,680	52,631
Accruals and other	14,536	15,432
Total deferred tax assets	937,930	799,237
Intangible assets	(1,499)	(2,518)
Right-of-use assets	(45,976)	(49,480)
Fixed assets	(7,576)	(2,670)
Less valuation allowance	(882,879)	(744,569)
Net deferred taxes	$ 0	$ 0