Quarterly Financial Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information (unaudited)
Revenues	$ 10,334	$ 1,257	$ 281	$ 147	$ 1,090	$ 91	$ 91	$ 208	$ 11,552
Total costs and expenses	21,691	18,432	15,760	18,055	13,834	12,381	9,348	8,086	73,939	43,649	38,555
Operating loss	(11,357)	(17,175)	(15,479)	(17,908)	(13,512)	(13,482)	(10,013)	(9,277)	(92,273)	(46,284)	(23,225)
Net loss	$ (22,406)	$ (24,096)	$ (27,357)	$ (18,414)	$ (13,512)	$ (13,482)	$ (10,013)	$ (9,277)	$ (92,273)	$ (46,284)	$ 53,815
Net loss per share, basic and diluted (in dollars per share)	$ (0.65)	$ (0.78)	$ (0.89)	$ (0.60)	$ (0.51)	$ (0.55)	$ (0.61)	$ (6.05)	$ (2.90)	$ (2.72)	$ 31.39