Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per 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 Information Disclosure [Abstract]
Revenues	$ 2,013	$ 1,486	$ 15,121	$ 32,234	$ 167	$ 171	$ 167	$ 171	$ 50,854	$ 676	$ 2,658
Operating expenses	15,118	14,618	13,552	14,215	10,869	9,737	11,359	9,138	57,503	41,103	44,068
Net income (loss)	$ (13,110)	$ (13,182)	$ 1,290	$ 17,852	$ (11,054)	$ (9,689)	$ (11,302)	$ (8,951)	$ (7,150)	$ (40,996)	$ (41,411)
Net Income (Loss) Per Share, Basic	$ (0.48)	$ (0.48)	$ 0.06	$ 0.82
Net Income (Loss) Per Share, Diluted	$ (0.48)	$ (0.48)	$ 0.06	$ 0.78
Net loss per share, Basic and Diluted					$ (0.52)	$ (0.55)	$ (0.64)	$ (0.51)	$ (0.29)	$ (2.20)	$ (2.84)