Stockholders' Equity - Additional Information (Detail)						6 Months Ended
	Jan. 30, 2019 shares	May 30, 2018 USD ($) $ / shares shares	Dec. 20, 2017 USD ($) $ / shares shares	Feb. 08, 2017 USD ($) $ / shares shares	Aug. 13, 2014 $ / shares shares	Jun. 30, 2019 $ / shares shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Stockholders Equity [Line Items]
Preferred stock, shares authorized						10,000,000
Preferred stock, par value | $ / shares						$ 0.001
Preferred stock, outstanding						0	0
Loan and Security Agreements | East West Bank | Revolving Credit Facility
Stockholders Equity [Line Items]
Borrowing capacity under credit facility | $				$ 16,000,000
Loan and Security Agreements | Venture Lending & Leasing VII and VIII, Inc. | Term Loan Facility
Stockholders Equity [Line Items]
Borrowing capacity under credit facility | $				$ 10,000,000
2014 Consultant Warrant
Stockholders Equity [Line Items]
Warrant exercise price | $ / shares					$ 10.70	$ 10.70
Warrants expiration date					Aug. 13, 2019
EWB Warrant
Stockholders Equity [Line Items]
Warrants issued to purchase common stock				40,000
Warrant exercise price | $ / shares				$ 3.64		3.64
Warrants expiration date				Feb. 08, 2022
Common stock warrants fair value | $				$ 125,000
EWB Warrant | Estimated Volatility
Stockholders Equity [Line Items]
Private placement warrant				78.8
EWB Warrant | Risk-Free Interest Rate
Stockholders Equity [Line Items]
Private placement warrant				1.94
EWB Warrant | Expected Life
Stockholders Equity [Line Items]
Private placement warrant				5 years
EWB Warrant | Dividend Yield
Stockholders Equity [Line Items]
Private placement warrant				0
VLL7 Warrant
Stockholders Equity [Line Items]
Warrants issued to purchase common stock				290,000
Warrant exercise price | $ / shares				$ 2.00
Warrants expiration date				Feb. 08, 2022
Common stock warrants fair value | $				$ 1,037,500
VLL7 Warrant | Estimated Volatility
Stockholders Equity [Line Items]
Private placement warrant				78.8
VLL7 Warrant | Risk-Free Interest Rate
Stockholders Equity [Line Items]
Private placement warrant				1.94
VLL7 Warrant | Expected Life
Stockholders Equity [Line Items]
Private placement warrant				5 years
VLL7 Warrant | Dividend Yield
Stockholders Equity [Line Items]
Private placement warrant				0
VLL8 Warrant
Stockholders Equity [Line Items]
Warrants issued to purchase common stock				290,000
Warrant exercise price | $ / shares				$ 2.00
Warrants expiration date				Feb. 08, 2022
Common stock warrants fair value | $				$ 1,037,500
VLL8 Warrant | Estimated Volatility
Stockholders Equity [Line Items]
Private placement warrant				78.8
VLL8 Warrant | Risk-Free Interest Rate
Stockholders Equity [Line Items]
Private placement warrant				1.94
VLL8 Warrant | Expected Life
Stockholders Equity [Line Items]
Private placement warrant				5 years
VLL8 Warrant | Dividend Yield
Stockholders Equity [Line Items]
Private placement warrant				0
Previous Lender
Stockholders Equity [Line Items]
Warrants to purchase common stock cancelled				400,000
2017 Consultant Warrant
Stockholders Equity [Line Items]
Warrants issued to purchase common stock				60,000
Warrant exercise price | $ / shares				$ 4.60
Warrants expiration date				Feb. 08, 2019
Common stock warrants fair value | $				$ 119,000
Issuance of common stock net exercise of warrants	10,449
2017 Consultant Warrant | Estimated Volatility
Stockholders Equity [Line Items]
Private placement warrant				78.8
2017 Consultant Warrant | Risk-Free Interest Rate
Stockholders Equity [Line Items]
Private placement warrant				1.22
2017 Consultant Warrant | Expected Life
Stockholders Equity [Line Items]
Private placement warrant				2 years
2017 Consultant Warrant | Dividend Yield
Stockholders Equity [Line Items]
Private placement warrant				0
Maximum | Common Stock
Stockholders Equity [Line Items]
Shares issued, price per share | $ / shares						$ 4.00
Maximum | Common Stock | 2014 Consultant Warrant
Stockholders Equity [Line Items]
Warrants issued to purchase common stock					85,000
Series A Participating Preferred Stock
Stockholders Equity [Line Items]
Preferred stock, shares authorized						40,000
Preferred stock, par value | $ / shares						$ 0.001
Preferred stock, outstanding						0	0
Series B Preferred Stock
Stockholders Equity [Line Items]
Preferred stock, shares authorized						5,000,000
Preferred stock, par value | $ / shares						$ 0.001
Preferred stock, outstanding						5,000,000	5,000,000
Shares issued, price per share | $ / shares			$ 4.00
Total purchase price payable | $			$ 20,000,000
Convertible preferred stock, terms of conversion						Each Share shall be convertible into the Company’s common stock (i) following the sixth (6th) anniversary of the initial closing of the Private Placement or (ii) if earlier, during the thirty (30) day period following the last trading day of any period of three (3) or more consecutive trading days that the closing market price of the Company’s common stock exceeds $10.00. Each Share is convertible at the option of the holder of shares of Series B Preferred Stock into such number of shares of the Company’s common stock
Convertible preferred stock, threshold closing market price of entity stock | $ / shares			$ 10.00
Percentage of beneficially ownership limitation in excess of outstanding common stock immediately after effect to applicable conversion			19.90%
Minimum conversion price | $ / shares						$ 3.27
Dividend payment terms						Each Share is entitled to a cumulative annual dividend of 5% for the first six (6) years following the issuance of such Share and 3% for each year thereafter, with the Company retaining the option to settle each year’s dividend after the tenth (10th) year in cash. The dividends accrue and are payable in kind upon such time as the Shares convert into the Company’s common stock.
Cumulative annual dividend for first six years						5.00%
Cumulative annual dividend for each year after sixth year						3.00%
Price per share distributable to stockholders | $ / shares						$ 4.00
Series B Preferred Stock | Common Stock
Stockholders Equity [Line Items]
Preferred stock shares convertible into common stock						5,337,500
Series B Preferred Stock | Private Placement at Initial Closing of Transaction | Tranche One
Stockholders Equity [Line Items]
Issuance of stock (in shares)			3,000,000
Shares issued, price per share | $ / shares			$ 4.00
Total purchase price payable | $			$ 12,000,000
Series B Preferred Stock | Private Placement at Second Closing of Transaction
Stockholders Equity [Line Items]
Issuance of stock (in shares)			2,000,000
Shares issued, price per share | $ / shares			$ 4.00
Series B Preferred Stock | Private Placement at Second Closing of Transaction | Tranche Two
Stockholders Equity [Line Items]
Issuance of stock (in shares)		2,000,000
Shares issued, price per share | $ / shares		$ 4.00
Total purchase price payable | $		$ 8,000,000
Series B Preferred Stock | Maximum
Stockholders Equity [Line Items]
Preferred stock, shares authorized								5,000,000
Preferred stock, issued			5,000,000