Summary of Significant Accounting Policies - Weighted Average Number of Common Shares Outstanding (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	$ (60,407)		$ (14,510)		$ (4,661)		$ (2,649)		$ (5,355)		$ (2,469)		$ (6,374)		$ (1,261)		$ (82,227)	$ (15,459)	$ (22,181)
Denominator:
Weighted average common shares outstanding																	97,111	90,870	89,165
Weighted average unvested common shares subject to repurchase																	(876)	(652)	(368)
Weighted average common shares outstanding—basic																	96,235	90,218	88,797
Effect of dilutive securities:
Options, warrants and restricted share awards
Weighted average common shares outstanding—diluted																	96,235	90,218	88,797
Net loss per common share:
Basic and diluted net loss per share	$ (0.62)	[1]	$ (0.15)	[1]	$ (0.05)	[1]	$ (0.03)	[1]	$ (0.06)	[1]	$ (0.03)	[1]	$ (0.07)	[1]	$ (0.01)	[1]	$ (0.85)	$ (0.17)	$ (0.25)

[1] Basic and diluted net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts will not necessarily equal the total for the year.