Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Beginning balance	$ 53,047	$ 104,319
Proceeds from 'at-the-market' offering, net, value	830
Stock-based compensation expense	1,592	1,421
Proceeds from direct offering, net, value	10,922
Proceeds from pre-funded common stock warrant from direct offering, net, value	13,032
Issuance of common stock warrant with direct offering, value	3,446
Unrealized gains on short-term investments	33	(75)
Employee withholdings ESPP	26	34
Net loss	(14,302)	(19,398)
Ending balance	68,626	86,950
ASU 2014-09 [Member]
Class of Stock [Line Items]
Cumulative effect adjustment of adoption ASU 2014-09		649
Common Stock [Member]
Class of Stock [Line Items]
Beginning balance	$ 2	$ 2
Beginning balance, Shares	24,835,951	24,390,033
Stock purchase under ESPP, Shares	23,970	8,696
Proceeds from "at-the-market" offering, net, shares	140,819
Proceeds from direct offering, net, value	$ 1
Proceeds from direct offering,net, Shares	2,095,039
Ending balance	$ 3	$ 2
Ending balance, Shares	27,095,779	24,697,944
Stock issuance due to warrant exercise, cashless, share		299,215
Additional Paid-In Capital [Member]
Class of Stock [Line Items]
Beginning balance	$ 492,493	$ 470,571
Proceeds from 'at-the-market' offering, net, value	830
Stock-based compensation expense	1,592	1,421
Proceeds from direct offering, net, value	10,921
Proceeds from pre-funded common stock warrant from direct offering, net, value	13,032
Issuance of common stock warrant with direct offering, value	3,446
Employee withholdings ESPP	26	34
Ending balance	522,340	472,026
Accumulated Other Comprehensive Income / (Loss) [Member]
Class of Stock [Line Items]
Beginning balance	(25)	(143)
Unrealized gains on short-term investments	33	(75)
Ending balance	8	(218)
Accumulated Deficit [Member]
Class of Stock [Line Items]
Beginning balance	(439,423)	(366,111)
Net loss	(14,302)	(19,398)
Ending balance	$ (453,725)	(384,860)
Accumulated Deficit [Member] | ASU 2014-09 [Member]
Class of Stock [Line Items]
Cumulative effect adjustment of adoption ASU 2014-09		$ 649