Formation and Business of the Company (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 01, 2019	Jan. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule Of Capitalization Equity [Line Items]
Maximum aggregate offering price of securities	$ 150,000,000
Net loss			$ (16,477,000)	$ (8,347,000)	$ (35,350,000)	$ (21,005,000)
Net cash used in operating activities					(32,310,000)	$ (23,944,000)
Cash, cash equivalents and investments			109,400,000		109,400,000
Accumulated deficit			$ (145,986,000)		$ (145,986,000)		$ (110,636,000)
Convertible Stock Converted into Shares of Common Stock
Schedule Of Capitalization Equity [Line Items]
Common shares issued upon conversion of convertible stock (shares)		9,629,405
Cantor Fitzgerald & Co
Schedule Of Capitalization Equity [Line Items]
Service fee as percentage of gross proceeds on sale of shares	3.00%
IPO
Schedule Of Capitalization Equity [Line Items]
Shares issued (shares)		8,050,000
Proceeds from issuance initial public offering, net		$ 125,400,000
Underwriter’s Option
Schedule Of Capitalization Equity [Line Items]
Shares issued (shares)		1,050,000
Common Stock
Schedule Of Capitalization Equity [Line Items]
Shares issued (shares)			246,416		605,030
Net proceeds from sale of securities after commission and before offering costs			$ 1,900,000		$ 4,700,000
Proceeds from sale of securities, commission			60,000		100,000
Proceeds from sale of securities, offering costs			$ 10,000		$ 200,000
Common Stock | At-The-Market Offering Under Sales Agreement | Cantor Fitzgerald & Co
Schedule Of Capitalization Equity [Line Items]
Maximum aggregate offering price of securities	$ 50,000,000