Statements of Comprehensive Loss Parenthetical (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Stock-based compensation expense associated with research and development expense	$ 1,022	$ 915	$ 2,352	$ 1,952
Stock-based compensation expense associated with general and administrative expense	$ 847	$ 680	$ 1,626	$ 1,361