Net Loss Per Share Attributable to Common Stockholders - Basic and Diluted Loss Per Share (Details) (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
Earnings Per Share [Abstract]
Net loss attributable to common stockholders									$ (40,426)	$ (31,390)	$ (69,108)
Weighted-average shares of common stock outstanding, excluding shares issued upon early exercise of unvested options and unvested restricted stock	52,185	51,544	51,153	50,798	50,486	50,163	49,763	49,384	51,427	49,929	39,824
Net loss per share attributable to common stockholders—basic and diluted	$ (0.21)	$ (0.21)	$ (0.17)	$ (0.20)	$ (0.15)	$ (0.15)	$ (0.17)	$ (0.16)	$ (0.79)	$ (0.63)	$ (1.74)