BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	30,000,000	30,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	142,658,037	132,376,670
Common stock, shares outstanding	142,658,037	132,376,670
Series 1 Preferred Stock
Preferred stock, stated value	$ 1,200	$ 1,200
Preferred stock, shares authorized	250,000	250,000
Preferred stock, shares issued	119,644	106,184
Preferred stock, shares outstanding	119,644	106,184
Preferred stock, liquidation preference	$ 143.6	$ 127.4