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Earnings Per Share - Additional Information (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Line Items]      
Convertible Class B common stock outstanding 2,601,996 2,711,811 2,699,710
Anti-dilutive stock options excluded from earnings per share 11,664 31,839 67,014