Summary of Significant Accounting Policies - Reverse Stock Split and Initial Public Offering (Details) $ / shares in Units, $ in Thousands				3 Months Ended	12 Months Ended
	Dec. 11, 2018 USD ($) $ / shares shares	May 07, 2018 USD ($) $ / shares shares	Apr. 20, 2018	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 31, 2018 shares
Reverse Stock Split
Stock split ratio			0.1504
Initial Public Offering
Convertible preferred stock warrants reclassified to additional paid-in capital | $		$ 900		$ 900
Proceeds from sale of common stock | $					$ 181,825	$ 0	$ 0
Common Stock
Initial Public Offering
Aggregate number of shares called by warrants (in shares)		100,558			76,762
IPO | Common Stock
Initial Public Offering
Shares issued during the period (in shares)		7,762,500
Shares issue price (in dollars per share) | $ / shares		$ 16.00
Net proceeds from initial public offering | $		$ 112,000
Shares issued upon conversion (in shares)		12,111,710						12,111,710
Follow-On Public Offering | Common Stock
Initial Public Offering
Shares issued during the period (in shares)	1,875,000
Shares issue price (in dollars per share) | $ / shares	$ 40.00
Proceeds from sale of common stock | $	$ 69,800
Selling Stockholders | Follow-On Public Offering
Initial Public Offering
Shares sold (in shares)	1,000,000