Quarterly report pursuant to Section 13 or 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 ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Feb. 28, 2017
Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Open market purchase of treasury shares   $ 1,772,000      
Open market purchase of treasury shares, fees   $ 9,000 $ 23,000    
Treasury Stock          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Open market purchase of common shares (in shares) 100 212,231 574,848    
Open market purchase of treasury shares $ 1,000 $ 1,772,000 $ 3,412,000    
Common Class A          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock repurchase program, authorized amount (up to)       $ 15,000,000 $ 10,000,000