Common Stock - Additional Information (Detail)			1 Months Ended					3 Months Ended
	Jul. 16, 2018 $ / shares shares	Dec. 26, 2017 $ / shares	Apr. 30, 2019 USD ($) $ / shares shares	Feb. 28, 2019 USD ($) shares	Aug. 21, 2018 USD ($) Affiliate $ / shares shares	Dec. 31, 2017 $ / shares shares	May 31, 2016 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Nov. 30, 2017 USD ($)
Class of Stock [Line Items]
Common stock, shares issued								139,000,000	126,485,000
Common stock sales agreement, aggregate offering amount | $								$ 139,000	$ 126,000
Gross proceeds from public offering | $								$ 7,512,000		$ 518,000
Settlement Warrants
Class of Stock [Line Items]
Warrants to purchase of common stock shares	2,000,000					2,000,000
Warrants exercise price | $ / shares	$ 3.00	$ 3.00				$ 3.00
Warrants exercisable period	1 year	1 year				1 year
Underwritten Public Offering
Class of Stock [Line Items]
Common stock, shares issued					2,500,000
Net proceeds from public offering | $					$ 5,100,000
Gross proceeds from public offering | $					$ 5,700,000
Number Of Affiliates Stock Holders | Affiliate					2
Percentage of affiliates stock holders					5.00%
Private Placement | PIPE Warrants
Class of Stock [Line Items]
Common stock, shares issued							17,642,482
Shares issued, price per share | $ / shares							$ 0.965
Warrants exercise price | $ / shares							$ 1.00
Warrants exercisable period							5 years
Gross proceeds from issuance of private placement | $							$ 17,000,000
Exchange of unit to share							1
Warrants exercisable shares of common stock exercised								803,108
Cash proceeds | $								$ 800,000
Warrants exercisable shares of common stock outstanding								16,839,375
Private Placement | PIPE Warrants | Director and Executive Officer
Class of Stock [Line Items]
Common stock, shares issued							544,039
Net offering proceeds to the company | $							$ 15,400,000
SVB Leerink
Class of Stock [Line Items]
Stock issued during period, shares									4,707,770
Net proceeds from issuance of common stock | $									$ 10,300,000
Common stock value available for issuance in connection with future stock sales | $								$ 32,000,000
SVB Leerink | Common Stock
Class of Stock [Line Items]
Stock issued during period, shares				12,515,559
Net proceeds from issuance of common stock | $				$ 7,500,000
New Enterprise Associates and Another Stockholder | Underwritten Public Offering
Class of Stock [Line Items]
Common stock, shares issued					2,000,000
Subsequent Event | Underwritten Public Offering
Class of Stock [Line Items]
Common stock, shares issued			21,739,131
Gross proceeds from common stock and warrant issed | $			$ 25,000,000
Percentage of beneficial ownership of voting securities			5.00%
Net proceeds from public offering | $			$ 22,800,000
Subsequent Event | Underwritten Public Offering | Common Stock
Class of Stock [Line Items]
Shares issued, price per share | $ / shares			$ 1.14
Net proceeds from public offering | $			$ 22,800,000
Subsequent Event | Underwritten Public Offering | Offering Warrants
Class of Stock [Line Items]
Warrants to purchase of common stock shares			25,000,000
Warrants exercise price | $ / shares			$ 1.25
Warrants exercisable period			2 years
Subsequent Event | Over Allotment Option | Offering Warrants
Class of Stock [Line Items]
Warrants to purchase of common stock shares			3,260,869
Subsequent Event | New Enterprise Associates | Underwritten Public Offering
Class of Stock [Line Items]
Common stock, shares issued			4,347,827
Subsequent Event | New Enterprise Associates | Underwritten Public Offering | Offering Warrants
Class of Stock [Line Items]
Warrants to purchase of common stock shares			4,347,827
Minimum | Underwritten Public Offering
Class of Stock [Line Items]
Shares issued, price per share | $ / shares					$ 2.26
Minimum | Subsequent Event | Underwritten Public Offering
Class of Stock [Line Items]
Shares issued, price per share | $ / shares			$ 1.14
Minimum | Subsequent Event | Underwritten Public Offering | Offering Warrants
Class of Stock [Line Items]
Shares issued, price per share | $ / shares			$ 0.01
Maximum
Class of Stock [Line Items]
Offering, issuance and sale of stocks and securities, shelf registration | $												$ 200,000,000
Maximum | SVB Leerink
Class of Stock [Line Items]
Common stock sales agreement, aggregate offering amount | $											$ 50,000,000
Common stock sales agreement commission, percentage											3.00%