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Subsequent Events (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Oct. 23, 2017
Oct. 16, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Subsequent Event [Line Items]            
Cash dividends declared per Class A and Class B share (in dollars per share)     $ 0.27 $ 0.25 $ 0.79 $ 0.7
Subsequent Event            
Subsequent Event [Line Items]            
Proceeds from disposal   $ 13.2        
Cash dividends declared per Class A and Class B share (in dollars per share) $ 0.27          
Dividends payable $ 6.7