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Defined Contribution Benefit Plan (Details) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Subsidiary
Y
Dec. 31, 2010
Dec. 31, 2009
Defined Contribution Benefit Plan        
Employer's matching contribution for every dollar the employees contribute (in cents) $ 50 $ 50    
Employer's matching contribution, maximum 7,500 3,000    
Employer's contribution as a percentage of employee's eligible compensation, maximum 3.00%      
Employer's matching contribution, vesting period (in years)   5    
Number of foreign subsidiaries   1    
Aggregate employer's contribution to pension plans   $ 2,100,000 $ 1,700,000 $ 900,000