Shareholders' Equity - Company's Shares of Common Stock Reserved for Issuance (Detail) - shares	Mar. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Total shares reserved	4,953,454	4,910,016
Warrants Acquired in Merger [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	750,840	750,840
Warrants Converted Pursuant to Merger [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	72,869	72,869
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
December 2018 SVB Warrants [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	11,332	11,332
Pre-funded 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	3,133,827	3,077,264
Issued and nonvested RSU's [Member]
Class of Warrant or Right [Line Items]
Total shares reserved	143,125	156,250