Unaudited Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Collaboration revenue	$ 1,668	$ 3,448	$ 45,587	$ 46,723
Operating expenses:
Research and development	50,854	40,479	136,886	97,095
General and administrative	12,024	10,412	37,209	31,955
Total operating expenses	62,878	50,891	174,095	129,050
Loss from operations	(61,210)	(47,443)	(128,508)	(82,327)
Interest income	409	320	1,301	584
Other income (loss)	(20)	19	(88)	46
Loss before income tax (benefit) provision	(60,821)	(47,104)	(127,295)	(81,697)
Income tax (benefit) provision	70	(7,531)	(158)	(11,629)
Net loss	$ (60,891)	$ (39,573)	$ (127,137)	$ (70,068)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (1.71)	$ (1.14)	$ (3.59)	$ (2.03)
Weighted average common shares outstanding:
Basic and diluted (in shares)	35,594	34,782	35,415	34,432