Stock Based Compensation - Summary of Stock Option Activity (Detail) - Employee Stock Option - 2013 Equity Incentive Plan and Inducement Plan	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Shares Subject to Outstanding Stock Options
Beginning balance (in shares) | shares	7,548,436
Options granted (in shares) | shares	1,689,360
Options exercised (in shares) | shares	(35,654)
Options canceled (in shares) | shares	(401,547)
Ending balance (in shares) | shares	8,800,595
Weighted-Average Exercise Price Per Share
Beginning balance (in dollars per share) | $ / shares	$ 32.21
Options granted (in dollars per share) | $ / shares	12.32
Options exercised (in dollars per share) | $ / shares	9.12
Options canceled (in dollars per share) | $ / shares	31.49
Ending balance (in dollars per share) | $ / shares	$ 28.52