CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net revenues from collaborators		$ 89,912	$ 47,159	$ 41,097
Operating expenses:
Research and development	[1]	390,635	382,392	276,495
General and administrative	[1]	199,365	89,354	60,610
Total operating expenses		590,000	471,746	337,105
Loss from operations		(500,088)	(424,587)	(296,008)
Other income (expense):
Interest income		12,236	8,308	5,859
Other (expense) income		(3,022)	6,171	76
Total other income		9,214	14,479	5,935
Net loss		$ (490,874)	$ (410,108)	$ (290,073)
Net loss per common share — basic and diluted		$ (5.42)	$ (4.79)	$ (3.45)
Weighted-average common shares used to compute basic and diluted net loss per common share		90,554	85,596	83,992
Comprehensive loss:
Net loss		$ (490,874)	$ (410,108)	$ (290,073)
Unrealized loss on marketable securities, net of tax		(2,886)	(30,833)	(44,394)
Reclassification adjustment for realized loss (gain) on marketable securities included in net loss		1,894	(6,977)
Comprehensive loss		$ (491,866)	$ (447,918)	$ (334,467)

[1] Stock-based compensation expenses included in operating expenses are as follows: Research and development $51,872 $42,946 $27,086 General and administrative 40,947 32,582 18,697