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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator
Net loss	$ (643)	$ (19,282)	$ (17,169)	$ (10,292)	$ (23,705)	$ (22,038)	$ (19,177)	$ (18,983)	$ (47,386)	$ (83,903)	$ (73,564)
Denominator
Weighted average common shares outstanding									80,558	42,715	4,267
Weighted average shares subject to repurchase									0	(3)	(94)
Weighted average shares outstanding, basic and diluted, in shares	100,714	90,370	65,449	65,369	65,215	37,320	34,018	33,973	80,558	42,712	4,173
Basic and diluted net loss per share	$ (0.01)	$ (0.21)	$ (0.26)	$ (0.16)	$ (0.36)	$ (0.59)	$ (0.56)	$ (0.56)	$ (0.59)	$ (1.96)	$ (17.63)