Shareholders' Equity - Company's Shares of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2018	Dec. 31, 2017
Class of Warrant or Right [Line Items]
Total shares reserved	4,468,840	4,072,391
Warrants from Mast acquired in Merger [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	750,840	1,152,231
Warrants Converted Pursuant to Merger [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	72,869	74,992
April 2017 SVB Warrants [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	24,725	24,725
June 2017 SVB Warrants [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	41,736	41,736
Prefunded Warrants [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	775,000	775,000
Stock Options Outstanding [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	2,634,295	1,916,832
Issued and Unvested RSU's [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	169,375	86,875