Stock-Based Compensation - Additional Information (Detail) - USD ($)	1 Months Ended				12 Months Ended
	Dec. 31, 2017	Nov. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 31, 2015	Jul. 23, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option grant, description					Options granted may be either incentive stock options or non-statutory stock options.
Common stock reserved for issuance	14,209,402		10,822,328		14,209,402	10,822,328
Grant date fair value of options granted, per share					$ 3.38	$ 2.72	$ 3.13
Grant date fair value of options vested					$ 4,800,000	$ 2,400,000	$ 782,000
Aggregate intrinsic value of options exercised					2,600,000	2,300,000
Unrecognized compensation cost	$ 9,900,000				$ 9,900,000
Weighted average period for recognition of compensation costs					2 years 8 months 12 days
RSUs Granted					56,022
Stock based compensation expenses					$ 5,319,000	2,907,000	1,066,000
Stock-based compensation expense capitalized					0	0	0
General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based compensation expenses					$ 4,064,000	$ 2,194,000	$ 754,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	149,636		151,240		149,636	151,240
Weighted average grant date fair value of RSUs granted					$ 8.02	$ 3.52	$ 0
Stock based compensation expenses					$ 0	$ 0	$ 0
Restricted Stock Units (RSUs) | General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based compensation expenses					$ 449,000	$ 532,000	$ 0
Restricted Stock Units (RSUs) | Board of Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs Granted	0	43,554		112,578
RSUs Released	18,964	0		0
Restricted Stock Units (RSUs) | Executive Officers
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs Granted		12,468	18,017
Restricted Stock Units (RSUs) | Consultant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs Granted			20,645
Two Thousand Eight Stock Option Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period of stock option plan					10 years
Exercisable period of stock option plan					4 years
Two Thousand Eight Stock Option Plan | Nonstatutory Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of fair value of common stock, percentage					85.00%
Two Thousand Eight Stock Option Plan | Ten Percent Stockholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period of stock option plan					5 years
Two Thousand Eight and Two Thousand Fifteen Stock Option Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Voting power Percentage					10.00%
Two Thousand Eight and Two Thousand Fifteen Stock Option Plan | Ten Percent Stockholder | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of fair value of common stock, percentage					110.00%
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance								667,670	285,621
Shares issued under ESPP					0	0