Income Taxes - Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax due at statutory rate	21.00%	21.00%	34.00%
State taxes, net of federal	2.80%	6.00%	5.10%
Stock-based compensation	1.40%	2.50%	6.40%
Foreign rate differential	(1.60%)	(4.30%)	(3.20%)
Federal and state credits	2.40%	2.50%	7.20%
Change in valuation allowance	(25.80%)	(27.60%)	(22.50%)
Other	0.50%	(0.10%)
Federal and state rate change	(0.70%)		(24.60%)
Research and orphan drug credit addback			(2.40%)
Total effective tax rate	0.00%	0.00%	0.00%