Income Taxes - Summary of Statutory Federal Tax Rate to Income or Loss before Taxes (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	34.00%	34.00%	34.00%
Change in valuation allowance	(7.00%)	(3.00%)	(8.00%)
Research and development credits	16.00%	4.00%	2.00%
Foreign loss not benefitted		(30.00%)	(26.00%)
Other Non-deductible expenses	(3.00%)	(5.00%)	(2.00%)
Change in rate differential	(38.00%)
Build-to-suit adjustments	(2.00%)
Total	0.00%	0.00%	0.00%