Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended		5 Months Ended	9 Months Ended		12 Months Ended
	Jul. 23, 2018	May 31, 2017	Mar. 31, 2017	Mar. 09, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Aug. 21, 2017	Jun. 30, 2017
Class of Stock [Line Items]
Proceeds from exercise of warrants						$ 3,119,000
Warrant issued					0		0
Warrant outstanding					0		0
Proceeds from issuance of common stock					$ 7,490,000	$ 4,000,000
Piper Jaffray & Co. [Member] | Equity Distribution Agreement and 2017 Prospectus [Member]
Class of Stock [Line Items]
Equity distribution agreement maximum value of common shares issuable								$ 8,500,000
Proceeds from issuance of common stock				$ 8,500,000	7,500,000
Offering costs					$ 232,000
Piper Jaffray & Co. [Member] | Equity Distribution Agreement and 2018 Prospectus [Member]
Class of Stock [Line Items]
Maximum value of additional common shares issuable	$ 9,800,000
Preferred Stock [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares									311,850
Proceeds from exercise of warrants							$ 3,100,000
Common Stock [Member] | Piper Jaffray & Co. [Member] | Equity Distribution Agreement and 2017 Prospectus [Member]
Class of Stock [Line Items]
Common stock number of shares issued				2,463,966
Common Stock [Member] | Piper Jaffray & Co. [Member] | Equity Distribution Agreement and 2018 Prospectus [Member]
Class of Stock [Line Items]
Common stock number of shares issued	0
Common Stock [Member] | OrbiMed Israel Partners Limited Partnership [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares		51,477
Common Stock [Member] | Peregrine Management II Ltd. [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares		10,299
Common Stock [Member] | Pontifax [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares		90,804
Preferred Class B [Member] | OrbiMed Israel Partners Limited Partnership [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares		6,458,628	978,561
Warrant exercise price		$ 0.41	$ 0.41
Proceeds from exercise of warrants		$ 2,600,000	$ 400,000
Preferred Class B [Member] | Peregrine Management II Ltd. [Member]
Class of Stock [Line Items]
Number of warrants exercised to purchase shares		192,454
Warrant exercise price		$ 0.41
Proceeds from exercise of warrants		$ 79,000