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Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Aug. 31, 2017	Jan. 31, 2019	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock				$ 8,000
2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance					1,223,163
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance					305,790
Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares issued				1,643,961		2,285,952
ATM Offering Program [Member] \| Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from issuance of common stock			$ 1,400,000
ATM Offering Program [Member] \| Cowen Company, LLC (Cowen) [Member] \| Sale Agreement [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Agreed upon value of sale of common stock per transaction							$ 35,000,000
ATM Offering Program [Member] \| Common Stock [Member] \| Cowen Company, LLC (Cowen) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period			2,711,158
Shares issued, price per share			$ 0.53
Private Placement [Member] \| VGO Fund [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Convertible preferred stock, common stock issued upon conversion		3,209,955
Private Placement [Member] \| VGO Fund [Member] \| Securities Purchase Agreement [Member] \| Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued, price per share	$ 13.25
Preferred stock, shares issued	2,285,952
Convertible preferred stock, terms of conversion			Each share of preferred stock is convertible into five shares of our common stock. In January 2019, the VGO Fund converted 641,991 shares of its preferred stock into 3,209,955 shares of common stock.
Convertible preferred stock, common stock issued upon conversion	5
Converted shares of preferred stock		641,991
Private Placement [Member] \| VGO Fund [Member] \| Maximum [Member] \| Securities Purchase Agreement [Member] \| Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	4,952,829
Private Placement [Member] \| Remaining Purchasers [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	6,488,205
Gross proceeds from transaction	$ 45,800,000
Private Placement [Member] \| Common Stock [Member] \| VGO Fund [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period	1,777,784
Shares issued, price per share	$ 2.65
Private Placement [Member] \| Common Stock [Member] \| Remaining Purchasers [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period	4,094,336
Shares issued, price per share	$ 2.65
Private Placement [Member] \| Common Stock [Member] \| Remaining Purchasers [Member] \| Maximum [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Warrants to purchase number of common stock, shares	1,535,376
Private Placement [Member] \| Warrants [Member] \| Remaining Purchasers [Member] \| Securities Purchase Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of warrant per share	$ 4.64
Warrants expiration date	Dec. 31, 2019