Financings - February 2015 Follow-on Underwritten Public Offering (Details) $ / shares in Units, $ in Millions	Oct. 28, 2016 USD ($) person $ / shares shares	Oct. 13, 2016 USD ($) $ / shares shares	Feb. 19, 2015 USD ($) person $ / shares shares	Feb. 10, 2014 $ / shares shares	May 05, 1998 shares	Dec. 31, 2016 shares
Class of Stock
Shares of common stock that may be purchased upon exercise of warrants						51,709,460
Common Stock
Class of Stock
Stock issued (in shares)	1,225,243	25,000,000	23,000,000	7,867,438	1,199,684
Common stock, price per share | $ / shares		$ 2.00	$ 3.75	$ 4.00
Gross proceeds from offering of common stock | $		$ 50.0	$ 86.3
Net proceeds from offering of common stock | $	$ 48.8		$ 80.6
Pre-funded Warrants
Class of Stock
Common stock, price per share | $ / shares				3.99
Warrant exercise price per share | $ / shares				$ 0.01
Pre-funded Warrants | Maximum
Class of Stock
Stock issued (in shares)				2,158,750
Investment Funds
Class of Stock
Number of people | person	2
Investment Funds | Directors
Class of Stock
Stock issued (in shares)	5,125,000
Common stock, price per share | $ / shares	$ 2.00
Investment Funds | Directors | Common Stock
Class of Stock
Stock issued (in shares)			5,333,333
Common stock, price per share | $ / shares			$ 3.75
Number of people | person			2
Baker Bros. Advisors LP
Class of Stock
Common stock, price per share | $ / shares			$ 0.47
Common stock held, shares			6,965,432
Shares of common stock that may be purchased upon exercise of warrants			20,316,327
Baker Bros. Advisors LP | Pre-funded Warrants
Class of Stock
Warrant exercise price per share | $ / shares			$ 0.01
Baker Bros. Advisors LP | Pre-funded Warrants | Maximum
Class of Stock
Shares of common stock that may be purchased upon exercise of warrants			22,151,052