Shareholder's Equity and Share-based Compensation - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Common stock, shares authorized	95,000,000	400,000,000
Common stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	5,000,000	10,000,000
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares outstanding	0
Dividends declared	$ 0
2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Exercise price determination description	The exercise price of an option is determined by the board of directors when the option is granted and may not be less than 85% of the fair market value of the shares on the date of grant, provided that the exercise price of an ISO is not less than 100% of the fair market value of the shares on the date of grant and the exercise price of any option granted to a 10% stockholder is not less than 110% of the fair market value of the shares on the date of grant.
Exercise price per share	85.00%
Equity incentive plan expiration period	10 years
2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options | Maximum
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Exercise price per share	100.00%
2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options | Maximum | 10% stockholder
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Exercise price per share	110.00%
2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Nonstatutory
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Equity incentive plan expiration period	10 years
2017 Equity Incentive Plan
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting period	4 years
Common shares reserved	3,294,919
Percentage of increase in common shares reserved	5.00%
Shares available for grant	2,512,698
2017 Equity Incentive Plan | Maximum
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Exercise price per share	100.00%
Options granted exercise period	10 years
2017 Equity Incentive Plan | Maximum | 10% stockholder
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Exercise price per share	110.00%
Share-based Compensation Award, Tranche One | 2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting percentage	25.00%
Vesting period	12 months
Share-based Compensation Award, Tranche Two | 2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting percentage	25.00%
Vesting period	12 months
Share-based Compensation Award, Tranche Three | 2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting percentage	25.00%
Vesting period	12 months
Share Based Compensation Award Tranche Four | 2015 Equity Incentive Plan and 2004 Equity Incentive Plan | Incentive Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting percentage	25.00%
Vesting period	12 months