Preferred Stock, Common Stock and Stockholders' Equity - Equity Incentive Award Plans - Additional Information (Detail)			6 Months Ended	12 Months Ended
	Apr. 26, 2018 shares	Apr. 27, 2016 shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 $ / shares shares	Dec. 31, 2018 $ / shares shares	Jan. 01, 2020 shares	Aug. 31, 2013 shares	May 31, 2007 shares
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan	6,286,425			7,678,152	6,881,051
Award plan expiration month and year	2028-02
Stockholders' equity, stock split, conversion ratio			0.75
Stock options exchanged			2,456,999
Stock options issued			1,842,746
Exercise price | $ / shares			$ 0.62
Share-based compensation expense | $			$ 84,000
Share-based compensation arrangement, vesting period			4 years
Options available for future grant				1,587,155	986,801
Intrinsic value, common stock price | $ / shares				$ 1.62	$ 2.48
Weighted Average Grant Date Fair Value Per Share, Granted | $ / shares				$ 0.91	$ 1.86
Options Granted [Member]
Sale Of Stock [Line Items]
Options granted, weighted average term				10 years
Number of stock options granted				829,500	886,000
Options Granted [Member] | Minimum [Member]
Sale Of Stock [Line Items]
Share-based compensation arrangement, vesting period				1 year
Options Granted [Member] | Maximum [Member]
Sale Of Stock [Line Items]
Share-based compensation arrangement, vesting period				4 years
2007 Equity Incentive Plan [Member]
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan				0				450,000
2013 Equity Incentive Award Plan [Member]
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan							510,000
Shares available for issuance, description				In addition, the number of shares of common stock available for issuance under the 2013 Plan will be annually increased on the first day of each fiscal year during the term of the 2013 Plan, beginning with the 2014 fiscal year, by an amount equal to the least of: (i) 300,000 shares; (ii) four percent of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) such other amount as the Company’s board of directors may determine.
Additional common stock reserved for issuance				4,073,526
Options available for future grant				1,587,155
2013 Equity Incentive Award Plan [Member] | Subsequent Event [Member]
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan						977,277
Options available for future grant						2,564,432
2013 Equity Incentive Award Plan [Member] | Annual Increase in Shares [Member]
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan				300,000
Reserve percentage for issuance of shares				4.00%
Amended and Restated Equity Incentive Plan 2013 [Member]
Sale Of Stock [Line Items]
Aggregate number of shares of common stock reserved under the plan	8,000,000	4,786,425
Shares available for issuance, description					In addition, beginning on January 1, 2019, the number of shares available for issuance will be annually increased on the first day of each fiscal year by that number of shares equal to the least of (a) four percent of the outstanding shares of common stock on the last day of the immediately preceding calendar year, and (b) such other amount determined by the Company’s board of directors. Notwithstanding the foregoing, the number of shares of common stock that may be issued or transferred pursuant to incentive stock options under the Restated Plan may not exceed an aggregate of 8,000,000 shares.
Increase in number of shares of common stock reserved under the plan	1,500,000	500,000
Award plan expiration year		2026
Amended and Restated Equity Incentive Plan 2013 [Member] | Annual Increase in Shares [Member]
Sale Of Stock [Line Items]
Reserve percentage for issuance of shares				4.00%