Stock-Based Compensation - Additional Information (Detail) - USD ($)		1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 04, 2019	Jun. 30, 2019	May 31, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2015	Jan. 01, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 2,713,000	$ 1,819,000	$ 7,628,000	$ 5,759,000
Restricted stock units, vested					0
Common stock, shares outstanding					46,673,977		46,673,977				39,547,558
Common stock, shares issued			74,166		46,673,977		46,673,977				39,547,558
Fair value of options granted					$ 15.29	$ 14.51	$ 11.35	$ 11.25
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from operations							$ 0	$ 0
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 1,322,000	$ 918,000	3,829,000	3,358,000
General and Administrative [Member] | Shareholder Consulting Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					98,000		197,000
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					1,391,000	$ 901,000	3,799,000	$ 2,401,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 0		1,194,000
Restricted stock units, vested		0	74,166
Common stock, shares issued			74,166
Restricted Stock Units [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense							590,000
Restricted Stock Units [Member] | Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense							$ 604,000
Director [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding					0		0
Director [Member] | Restricted Stock Units [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 123,000		$ 164,000
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										6,086,907	4,900,481
Options granted					20,000	165,000	1,218,000	897,500
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] | Director [Member] | Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted							3 years
2014 Equity Incentive Plan [Member] | Director [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted							1 year
Restricted stock units, approved and granted	24,000						24,000
Restricted stock units, grant date fair value	$ 20.47
Compensation expense					$ 123,000		$ 164,000
2014 Equity Incentive Plan [Member] | Director [Member] | Minimum [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting date	Jun. 04, 2020
2014 Equity Incentive Plan [Member] | Director [Member] | Share-based Payment Arrangement, Nonemployee [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, approved and granted							24,000
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, approved and granted				215,000
Restricted stock units, grant date fair value				$ 16.10
2014 Equity Incentive Plan [Member] | Employee And Nonemployee Consultants [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%
Vesting period of awards granted							36 months
2014 Equity Incentive Plan [Member] | Employee And Nonemployee Consultants [Member] | Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted							4 years
Non-employee Director Compensation Policy [Member] | Director [Member] | Share-based Payment Arrangement, Nonemployee [Member] | Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, approved and granted		6,000