Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Data (unaudited)
Total revenue					$ 4	$ 9	$ 15	$ 16		$ 44	$ 6,098
Loss from operations	(9,315)	(7,709)	(9,534)	(10,160)	(9,928)	(11,087)	(11,571)	(8,323)	(36,718)	(40,909)	(23,482)
Net loss	$ (9,713)	$ (8,063)	$ (9,814)	$ (10,408)	$ (10,236)	$ (11,329)	$ (11,809)	$ (8,574)	$ (37,998)	$ (41,948)	$ (23,509)
Basic and diluted net loss per common share (in dollars per share)	$ (0.29)	$ (0.24)	$ (0.30)	$ (0.32)	$ (0.31)	$ (0.47)	$ (0.49)	$ (0.55)	$ (1.15)	$ (1.73)	$ (11.22)
Weighted average common shares outstanding used to calculate basic and diluted net loss per common share (in shares)	32,987,354	32,981,725	32,981,396	32,966,471	32,925,194	24,263,745	24,189,819	15,607,379	32,979,288	24,270,407	2,095,950