Summary of Significant Accounting Policies - Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator
Net loss	$ (35,617)	$ (10,852)	$ (13,332)	$ (21,055)	$ (643)	$ (19,282)	$ (17,169)	$ (10,292)	$ (80,856)	$ (47,386)	$ (83,903)
Denominator
Weighted average common shares outstanding									108,568	80,558	42,715
Weighted average shares subject to repurchase									0	0	(3)
Weighted average shares outstanding, basic and diluted	127,869	104,682	100,876	100,809	100,714	90,370	65,449	65,369	108,568	80,558	42,712
Basic and diluted net loss per share	$ (0.28)	$ (0.10)	$ (0.13)	$ (0.21)	$ (0.01)	$ (0.21)	$ (0.26)	$ (0.16)	$ (0.74)	$ (0.59)	$ (1.96)