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Share-Based Compensation Plans (Details) - shares
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock awarded in period (in shares) 491,769 946,570
Class B Common Stock Bonus    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock awarded in period (in shares)   513,570
Restricted stock requisite service period 3 years  
Class B Common Stock Key Employees    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock requisite service period 10 years 10 years