CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) shares in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Net revenues from collaborators		$ 47,159,000	$ 41,097,000	$ 50,561,000
Operating expenses:
Research and development	[1]	382,392,000	276,495,000	190,249,000
In-process research and development				220,766,000
General and administrative	[1]	89,354,000	60,610,000	44,526,000
Total operating expenses		471,746,000	337,105,000	455,541,000
Loss from operations		(424,587,000)	(296,008,000)	(404,980,000)
Other income:
Interest income		8,308,000	5,859,000	2,559,000
Other income		6,171,000	76,000	1,817,000
Total other income		14,479,000	5,935,000	4,376,000
Loss before income taxes		(410,108,000)	(290,073,000)	(400,604,000)
Benefit from income taxes		0	0	40,209,000
Net loss		$ (410,108,000)	$ (290,073,000)	$ (360,395,000)
Net loss per common share — basic and diluted		$ (4.79)	$ (3.45)	$ (4.85)
Weighted-average common shares used to compute basic and diluted net loss per common share		85,596	83,992	74,278
Comprehensive loss:
Net loss		$ (410,108,000)	$ (290,073,000)	$ (360,395,000)
Unrealized (loss) gain on marketable securities, net of tax		(30,833,000)	(44,394,000)	31,127,000
Reclassification adjustment for realized gain on marketable securities included in net loss		(6,977,000)		(2,081,000)
Comprehensive loss		$ (447,918,000)	$ (334,467,000)	$ (331,349,000)

[1] Non-cash stock-based compensation expenses included in operating expenses are as follows: Research and development $42,946 $27,086 $18,233 General and administrative 32,582 18,697 14,828