Statements of Comprehensive Loss Parenthetical (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Stock-based compensation expense associated with research and development expense	$ 940	$ 1,022	$ 2,398	$ 2,352
Stock-based compensation expense associated with general and administrative expense	$ 900	$ 847	$ 1,692	$ 1,626