Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Common stock, par value   $ 0.01
Common stock, shares authorized 0 78,837,300
Common stock, shares issued 0 57,182,730
Common stock, shares outstanding 0 55,118,673
Class A    
Common stock, par value $ 0.01  
Common stock, shares authorized 400,000,000 0
Common stock, shares issued 14,787,264 0
Common stock, shares outstanding 14,429,567 0
Treasury stock, shares 357,697 0
Class B    
Common stock, par value $ 0.01  
Common stock, shares authorized 200,000,000 0
Common stock, shares issued 55,639,613 0
Common stock, shares outstanding 53,492,633 0
Treasury stock, shares 2,146,980 2,064,057