Stock Plans (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Stock options
     
Summary of stock option activity      
Outstanding at beginning of the period (in shares) 1,277,294    
Granted (in shares) 226,341 358,000 165,000
Granted to replace options assumed in the MediaMind acquisition (in shares) 1,153,300    
Exercised (in shares) (16,206)    
Forfeited (in shares) (91,840)    
Expired (in shares) (64,045)    
Outstanding at the end of the period (in shares) 2,484,844 1,277,294  
Exercisable at end of the period (vested) (in shares) 993,281    
Weighted-Average Exercise Price      
Outstanding at the beginning of the period (in dollars per share) $ 19.64    
Granted (in dollars per share) $ 20.45    
Granted to replace options assumed in the MediaMind acquisition (in dollars per share) $ 16.63    
Exercised / vested (in dollars per share) $ 13.09    
Forfeited (in dollars per share) $ 20.78    
Expired (in dollars per share) $ 40.08    
Outstanding at the end of the period (in dollars per share) $ 17.79 $ 19.64  
Exercisable at end of the period (vested) (in dollars per share) $ 16.37    
2011 Plan
     
Stock Plans      
Maximum number of shares of common stock with respect to which awards may be granted 2,779,466    
Term of stock option grants (in years) P10Y    
Vesting period (in years) 4 years    
Number of shares of common stock available for future grant 2,719,406