Income Taxes - Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax due at statutory rate	21.00%	34.00%	34.00%
State taxes, net of federal	6.00%	5.10%	4.20%
Stock-based compensation	2.50%	6.40%	(1.00%)
Foreign rate differential	(4.30%)	(3.20%)	(3.10%)
Federal and state credits	2.50%	7.20%	10.30%
Change in valuation allowance	(27.60%)	(22.50%)	(41.50%)
Other	(0.10%)
Federal and state rate change		(24.60%)
Research and orphan drug credit addback		(2.40%)	(2.90%)
Total effective tax rate	0.00%	0.00%	0.00%