Stock-Based Compensation and Warrants - Summary of Activity Under Stock Incentive Plans (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Outstanding, Beginning Balance | shares	288,129
Shares Subject to Outstanding, Options granted | shares	287,577
Shares Subject to Outstanding, Options exercised | shares	(44,354)
Shares Subject to Outstanding, Options cancelled and forfeited | shares	(6,158)
Shares Subject to Outstanding, Ending Balance | shares	525,194
Shares Subject to Outstanding, Vested and expected to vest | shares	510,184
Shares Subject to Outstanding, Exercisable | shares	149,624
Weighted-Average Exercise Price Per Share, Outstanding Beginning Balance | $ / shares	$ 63.62
Weighted-Average Exercise Price Per Share, Options granted | $ / shares	6.76
Weighted-Average Exercise Price Per Share, Options exercised | $ / shares	21.73
Weighted-Average Exercise Price Per Share, Options cancelled and forfeited | $ / shares	41.54
Weighted Average Exercise Price Per Share, Outstanding Ending Balance | $ / shares	36.34
Weighted Average Exercise Price Per Share, Vested and expected to vest | $ / shares	36.90
Weighted Average Exercise Price Per Share, Exercisable | $ / shares	$ 84.00
Aggregate Intrinsic Value, Outstanding Balance | $	$ 63
Aggregate Intrinsic Value, Vested and expected to vest | $	61
Aggregate Intrinsic Value, Exercisable | $	$ 1