CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 4,403		$ 7,284
Operating expenses:
Research and development	16,588	$ 7,424	25,849	$ 9,516
General and administrative	9,774	3,716	18,945	4,786
Total operating expenses	26,362	11,140	44,794	14,302
Loss from operations	(21,959)	(11,140)	(37,510)	(14,302)
Other income (expense):
Interest income	498	47	965	68
Interest expense	(1)	(6,266)	(5)	(6,266)
Other income	570	1	571	1
Total other income (expense)	1,067	(6,218)	1,531	(6,197)
Net loss	(20,892)	(17,358)	(35,979)	(20,499)
Series A preferred stock dividend		(532)		(1,089)
Net loss attributable to common stockholders	$ (20,892)	$ (17,890)	$ (35,979)	$ (21,588)
Net loss per share, basic and diluted	$ (0.48)	$ (2.27)	$ (0.84)	$ (3.20)
Weighted-average shares outstanding, basic and diluted	43,249,349	7,890,029	42,859,708	6,737,169
Comprehensive loss:
Net loss	$ (20,892)	$ (17,358)	$ (35,979)	$ (20,499)
Foreign currency translation adjustments	680		681
Unrealized gain on available-for-sale securities, net	(133)		(44)
Comprehensive loss	$ (20,345)	$ (17,358)	$ (35,342)	$ (20,499)