Annual report pursuant to Section 13 and 15(d)

Stock Incentive Plan - Additional Information (Details)

v3.8.0.1
Stock Incentive Plan - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 01, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2011
Dec. 31, 2014
Dec. 31, 2010
May 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Granted (in shares)   390,353 665,000 747,100        
Total intrinsic value of options exercised   $ 808 $ 575 $ 1,024        
Exercise of stock options, net (in shares)   401,031 320,892 332,600        
Exercised (in usd per share)   $ 4.935 $ 3.286 $ 3.467        
Proceeds from exercises of options   $ 1,977 $ 1,054 $ 1,153        
Weighted average exercise price (in usd per share)   $ 7.367 $ 7.300 $ 7.740   $ 7.721    
Stock-based compensation expense   $ 1,293 $ 1,462 $ 1,769        
Stock Options                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Options vesting period         5 years      
Ratable vesting percentage   33.33%            
Exercise term upon termination     90 days          
Weighted average exercise price (in usd per share)   $ 2.93 $ 2.60 $ 3.03        
Non-vested stock options unrecognized compensation expense   $ 2,768            
Non-vested stock options unrecognized compensation expense, weighted-average period of recognition   2 years 3 months 18 days            
Stock Options | Minimum                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Options vesting period   3 years            
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 Stock Incentive Plan                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares reserved for future issuance (in shares)             28,500,000  
Common stock, par value (in dollars per share)             $ 0.0001  
2000 Stock Incentive Plan | Stock Options | Maximum                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Options vesting period   10 years            
2010 Stock Incentive Plan | Stock Options                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares available for grant (in shares)   7,100,394            
Options vesting period   10 years            
2010 Stock Incentive Plan | Class A Common Stock                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares reserved for future issuance (in shares)               10,000,000
Granted (in shares)   3,509,957            
2017 Employee Stock Purchase Plan | Stock Options                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of allocated shares (up to) (in shares) 100,000              
Discount from fair value of stock 5.00%              
Stock issued during period (in shares)   0            
Amount withheld for future purchases   $ 38