Annual report pursuant to Section 13 and 15(d)

Stock Incentive Plan - Narrative (Details)

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Stock Incentive Plan - Narrative (Details) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock, par value (in dollars per share)     $ 0.0001  
Granted (in shares) 598,360 [1] 706,644 [1] 257,500  
Granted (in usd per share) $ 9.101 [1] $ 11.782 [1] $ 12.520  
Weighted-average remaining contractual life of all options expected to vest 8 years 4 months 10 days      
Total intrinsic value of options exercised $ 4,224,406 $ 12,829,917 $ 31,757,430  
Exercise of stock options (in shares) 682,619 1,306,145 2,790,188  
Exercised (in usd per share) $ 3.037 $ 2.651 $ 2.305  
Proceeds from exercises of options 2,073,227 3,462,679 6,407,804  
Weighted average exercise price (in usd per share) $ 7.528 $ 6.794 $ 5.151 $ 4.177
Stock-based compensation expense 2,799,403 3,351,142 2,865,706  
Class A Common Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock, par value (in dollars per share) $ 0.0001 $ 0.0001    
2000 Plan
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance     28,500,000  
2010 Plan | Class A Common Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance     10,000,000  
Granted (in shares) 1,562,504      
Stock Options
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expiration period 10 years      
Options vesting period 5 years      
Share-based compensation vesting percentage 20.00%      
Ratable vesting percentage 5.00%      
Exercise term upon termination 90 days      
Weighted average exercise price (in usd per share) $ 3.66 $ 4.03 $ 5.59  
Non-vested stock options unrecognized compensation expense $ 5,984,747      
Non-vested stock options unrecognized compensation expense, weighted-average period of recognition 2 years 10 months 17 days      
Stock Options | 2010 Plan
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares available for grant (in shares) 8,535,127      
[1] Grants are related to the 2010 Plan.