Purchase Agreement, Controlled Equity Offering and Public Offering of Common Stock - Schedule of Amounts Offered and Sold under 2017 Sales Agreement (Detail) - Stifel, Nicolaus & Company, Incorporated [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold	0
2017 Sales Agreement [Member]
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold		15,998,500
Average price per share		$ 3.13
Gross proceeds		$ 50,000
Commissions earned by Cantor		1,500
Other issuance costs		$ 97