Description of the Business - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2018	Jun. 30, 2017	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]
Proceeds from underwritten public offering				$ 56,486	$ 26,073	$ 12,518
Underwriters partially exercised an overallotment option, additional gross proceeds				1,086	2,607	876
Proceeds from underwritten public offering after fees and commissions		$ 56,500
Proceeds from sale of common stock, net				$ 56,486	$ 26,223	$ 12,518
Sale of Stock, Number of Shares Issued in Transaction		3,450,000
Subsidiary or Equity Method Investee, Cumulative Proceeds Received on All Transactions		$ 57,800
Overallotment option
Subsidiary, Sale of Stock [Line Items]
Sale of Stock, Number of Shares Issued in Transaction		450,000
At the market equity offering
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued (in shares)					1,331,539	960,400
Proceeds from underwritten public offering after fees and commissions					$ 26,100	$ 12,500
Proceeds from sale of common stock, net	$ 40,000		$ 40,000