BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowances	$ 47	$ 47
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, authorized (in shares)	10,000,000	76,894,620
Preferred stock, issued (in shares)	0	76,235,050
Preferred stock, outstanding (in shares)	0	76,235,050
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized (in shares)	200,000,000	110,000,000
Common stock, issued (in shares)	23,401,675	1,272,360
Common stock, outstanding (in shares)	23,401,675	1,272,360