Income taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$ 439,839		$ 298,701
Tax credit carryforwards (federal and state)	213,810		167,517
Capitalized license fees and research and development expenses	16,295		18,083
60 Binney Street lease	0		42,059
Deferred revenue	15,119		21,442
Stock-based compensation	46,111		31,858
Lease liabilities	52,631		0
Accruals and other	15,432		8,886
Total deferred tax assets	799,237	$ 5,300	588,546
Intangible assets	(2,518)		(3,579)
Right-of-use assets	(49,480)		0
Fixed assets	(2,670)		(41,472)
Less valuation allowance	(744,569)		(543,495)
Net deferred taxes	$ 0	$ (1,800)	$ 0