Net Loss Per Share - Weighted Average Number of Common Shares Outstanding (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator:
Net loss for basic earnings per share	$ (3,041)	$ (14,510)	$ (12,609)	$ (21,820)
Decrease in fair value of warrants	(513)	0	(520)	0
Diluted net loss applicable to common stockholders	$ (3,554)	$ (14,510)	$ (13,129)	$ (21,820)
Denominator:
Weighted average common shares outstanding	98,126	97,318	97,864	96,940
Weighted average unvested common shares subject to repurchase	(735)	(937)	(824)	(894)
Weighted average common shares outstanding—basic	97,391	96,381	97,040	96,046
Effect of dilutive securities:
Warrants	938	0	218	0
Weighted average common shares outstanding—diluted	98,329	96,381	97,258	96,046
Net loss per share:
Basic	$ (0.03)	$ (0.15)	$ (0.13)	$ (0.23)
Diluted	$ (0.04)	$ (0.15)	$ (0.13)	$ (0.23)