Income taxes - Reconciliation of Income Tax Provision (Benefit) (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	21.00%	34.00%
State income tax, net of federal benefit	3.70%	5.10%	4.30%
Permanent differences	(1.10%)	0.90%	2.70%
Research and development credit	5.40%	6.50%	12.80%
Foreign differential	(3.70%)	(4.40%)	(6.90%)
Federal tax rate change	0	0.001	(0.316)
Other	0.40%	(0.10%)	(0.80%)
Change in valuation allowance	(25.60%)	(29.10%)	(14.60%)
Effective income tax rate benefit (expense)	0.10%	0.00%	(0.10%)