Net Loss Per Share Attributable to Common Stockholders (Potential Common Shares Outstanding Excluded from Computation of Diluted Net Loss Per Share) (Details) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	8,885	8,753	34,113
Convertible preferred stock
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	0	0	25,171
Stock options to purchase common stock
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	8,196	8,174	6,256
Convertible notes
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	0	0	2,328
Unvested restricted stock units
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	381	248	0
Warrants to purchase common stock
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	308	331	143
Convertible preferred stock warrants
The potential common shares outstanding that were excluded from the computation of diluted net loss per share attributable to common stockholders
Total	0	0	215