Net Loss Per Share (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Net Loss Per Share      
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares) 11,579 14,589 13,805
Restricted stock
     
Net Loss Per Share      
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares) 1,918 3,543 3,672
Stock options
     
Net Loss Per Share      
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares) 2,490 3,299 4,713
Weighted average exercise price (in dollars per share) $ 4.42 $ 4.74 $ 4.56
ESPP
     
Net Loss Per Share      
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares) 42 67 76
Convertible notes
     
Net Loss Per Share      
Anti-dilutive securities excluded from computation of diluted net loss per share (in shares) 7,129 7,680 5,344