Stock Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 01, 2019	Sep. 01, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 31, 2017	Feb. 28, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate proceeds from shares issued under the plan						$ 829
Stock-based compensation				$ 1,987	$ 719	5,199	$ 1,668
Total intrinsic value of options exercised						$ 1,400	$ 1,600
Weighted-average grant date fair value per share for options granted						$ 14.70	$ 8.51
Remaining shares subject to repurchase				72,577		72,577		151,289
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities				$ 48		$ 48		$ 112
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance									3,225,346	2,446,323
Stock option vesting period						4 years
Stock options expiration period						10 years
Number of additional shares available for future grant				0		0
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance			3,186,205
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	4.00%		4.00%
Maximum shares of common stock may be issued			20,887,347
Number of additional shares added to the plan	1,494,341
Number of shares outstanding available for future grant				2,643,252		2,643,252
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance			353,980
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	1.00%		1.00%
Number of additional shares added to the plan	373,585
Number of shares outstanding available for future grant				672,331		672,331
Maximum shares allowed to be issued under ESPP			4,778,738
Purchase of common stock through payroll deductions expressed in percentage of fair market value		85.00%
Common stock offering period		6 months
Number of shares issued to the plan						55,234
Aggregate proceeds from shares issued under the plan						$ 800
Stock-based compensation				$ 100		100
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense				$ 22,000		$ 22,000
Unrecognized compensation expense estimated to be recognized over period						2 years 7 months 6 days