Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Share Repurchase Program (Details)

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Summary of Significant Accounting Policies - Share Repurchase Program (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Feb. 28, 2017
Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Treasury stock, shares, acquired 574,848 1,298,418    
Treasury stock, value, acquired $ 3,412,000 $ 6,387,000    
Repurchase fees $ 23,000 $ 52,000    
Treasury Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Treasury stock, shares, acquired (574,848) (1,298,418)    
Treasury stock, value, acquired $ 3,412,000 $ 6,387,000    
Class A Common Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock repurchase program, authorized amount (up to)     $ 15,000,000 $ 10,000,000