Stockholders' Equity - Employee Stock Purchase Plan and Equity Incentive Award Plan and Stock-Based Compensation - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 26, 2018	May 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 03, 2017	May 02, 2017
Sale Of Stock [Line Items]
Payroll withholdings from employees			$ 47,000	$ 135,000	$ 47,000	$ 135,000
Issuance of common stock under employee stock purchase plan					28,869	75,529
Aggregate number of shares of common stock reserved under the plan	6,286,425
Award plan expiration month and year	2028-02
Stock options granted			0	0
Unrecognized compensation costs			$ 2,200,000		$ 2,200,000
Weighted average period					1 year 2 months 26 days
Amended and ESPP Plan [Member]
Sale Of Stock [Line Items]
Increase in number of shares of common stock reserved under the plan		100,000
Number of shares increased annually to common stock shares reserved for issuance							100,000	30,000
Aggregate number of shares of common stock reserved under the plan		1,250,000
Award plan expiration year		2027
Amended and Restated Equity Incentive Plan 2013 [Member]
Sale Of Stock [Line Items]
Increase in number of shares of common stock reserved under the plan	1,500,000
Aggregate number of shares of common stock reserved under the plan	8,000,000
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 Equity Incentive Plan may not exceed an aggregate of 8,000,000 shares.
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%