Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value per share (in USD per share)	$ 0.001
Preferred stock authorized (shares)	28,322,761	14,120,883
Preferred stock issued (shares)	25,975,346	14,120,883
Preferred stock outstanding (shares)	25,975,346	14,120,883
Preferred stock, liquidation value	$ 109,800	$ 59,300
Common stock, par value per share (in USD per share)	$ 0.0001	$ 0.0001
Common stock authorized (shares)	55,000,000	36,500,000
Common stock issued (shares)	5,298,593	5,280,058
Common stock outstanding (shares)	5,298,593	5,280,058
Series A Convertible Preferred Stock
Preferred stock, par value per share (in USD per share)	$ 0.001	$ 0.001
Preferred stock authorized (shares)	14,300	14,300
Preferred stock issued (shares)	14,300	14,300
Preferred stock outstanding (shares)	14,300	14,300
Preferred stock, liquidation value	$ 14,300	$ 14,300
Series B Convertible Preferred Stock
Preferred stock, par value per share (in USD per share)	$ 0.001	$ 0.001
Preferred stock authorized (shares)	14,106,583	14,106,583
Preferred stock issued (shares)	14,106,583	14,106,583
Preferred stock outstanding (shares)	14,106,583	14,106,583
Preferred stock, liquidation value	$ 45,000	$ 45,000
Series C Convertible Preferred Stock
Preferred stock, par value per share (in USD per share)	$ 0.001	$ 0.001
Preferred stock authorized (shares)	14,201,878	0
Preferred stock issued (shares)	11,854,463	0
Preferred stock outstanding (shares)	11,854,463	0
Preferred stock, liquidation value	$ 50,500	$ 0