Stock-Based Compensation - Additional Information (Detail)		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) PerformanceTarget $ / shares shares	Sep. 30, 2018 USD ($) PerformanceTarget $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) PerformanceTarget $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2015 shares	Jan. 01, 2018 shares	Dec. 31, 2017 shares	Jan. 01, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $		$ 1,819,000	$ 2,094,000	$ 5,759,000	$ 4,520,000
Fair value of options granted | $ / shares		$ 14.51	$ 10.00	$ 11.25	$ 11.61
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from operations | $				$ 0	$ 0
ASU 2016-09 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment to stockholders' equity | $									$ 45,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of performance targets achieved | PerformanceTarget	0	0		0
Restricted stock units, vested	0
Compensation expense | $	$ 0
Non-employee Consultants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair values of outstanding options | $ / shares	$ 20.48	$ 20.48	$ 8.31	$ 20.48	$ 8.31
2014 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options that may be granted after the tenth anniversary of the 2014 Plan				0
Annual increases in number of shares reserved for issuance as a percentage of shares of capital stock outstanding through January 1, 2024						3.00%
Share-based compensation arrangement by share-based payment award, number of shares authorized							4,900,481	3,920,613
Options granted		165,000	415,000	897,500	1,253,500
2014 Equity Incentive Plan [Member] | Share-Based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of vested shares on first anniversary of grant date				25.00%
2014 Equity Incentive Plan [Member] | Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted				36 months
2014 Equity Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of awards granted				10 years
2014 Equity Incentive Plan [Member] | Maximum [Member] | Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized						30,000,000
2014 Equity Incentive Plan [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted				4 years
2014 Equity Incentive Plan [Member] | Board of Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted				3 years
2014 Equity Incentive Plan [Member] | Executive Officers [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, granted		83,791		83,791
Restricted stock units, grant date fair value | $ / shares		$ 20.21