Income Taxes - Schedule of Reconciliation of Income Tax Expense Computed at Statutory Corporate Income Tax Rate to Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Effective Income Tax Rate
Income tax expense at statutory rate	10.30%	10.30%	10.30%
State income tax, net of federal benefit	1.30%	0.10%	0.00%
Nondeductible expenses	1.60%	0.00%	0.00%
Foreign rate differential	(3.30%)	(1.40%)	1.80%
Statutory to US GAAP permanent differences	6.60%	0.00%	0.00%
Stock-based compensation	(4.90%)	(1.40%)	(1.10%)
Research credits	3.10%	0.60%	0.00%
Change in valuation allowance	(16.80%)	(8.20%)	(10.10%)
Effective income tax rate	(2.10%)	0.00%	0.90%