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CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Class A Common Stock    
Common stock, par value (USD per share) $ 0.001 $ 0.001
Class B Common Stock    
Common stock, par value (USD per share) 0.001 0.001
Class C Common Stock    
Common stock, par value (USD per share) $ 0.001 $ 0.001