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Capital Stock (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2018
Aug. 16, 2018
Dec. 31, 2017
Class of Stock [Line Items]      
Class A common stock   $ 173.3  
Share-based Compensation Arrangement by Share-based Payment Award, Aggregate Value of Shares Issued $ 0.5    
Class A Common Stock      
Class of Stock [Line Items]      
Common stock outstanding (in shares) 38,107,555   33,560,202
Stock repurchased and retired (in shares) 0    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period 11,389    
Class B Common Stock      
Class of Stock [Line Items]      
Common stock outstanding (in shares) 22,501,994   22,905,357