Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.01	$ 0.01
Convertible preferred stock, authorized	308,593,000	308,593,000
Convertible preferred stock, issued	0	304,958,000
Convertible preferred stock, outstanding	0	304,958,000
Liquidation preference	$ 0
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	17,579,000	231,000
Common stock, shares outstanding	17,579,000	231,000