Statements of Comprehensive Loss Parenthetical (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Stock-based compensation expense associated with research and development expense	$ 1,027	$ 923	$ 3,379	$ 2,875
Stock-based compensation expense associated with general and administrative expense	$ 723	$ 669	$ 2,349	$ 2,030