BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
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	142,398,936	142,658,037
Common stock, shares outstanding	142,398,936	142,658,037
Series 1 Preferred Stock
Preferred stock, stated value	$ 1,200	$ 1,200
Preferred stock, shares authorized	250,000	250,000
Preferred stock, shares issued	123,268	119,644
Preferred stock, shares outstanding	123,268	119,644
Preferred stock, liquidation preference	$ 147.9	$ 143.6