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Shareholders' Equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 23, 2017
Sep. 30, 1999
Stockholders Equity Note [Line Items]          
Preferred stock outstanding 0 0      
Maximum aggregate offering price under sales agreement       $ 250,000,000  
Net proceeds from the sale of Common stock $ 247,744,000        
Direct stock issuance costs $ 311,000        
Number of shares authorized to be repurchased         7,500,000
Treasury stock, shares repurchased 0 0 0    
Cost of repurchased shares $ 87,440,000 $ 114,425,000      
Remaining number of shares authorized to be repurchased 1,129,087        
Common Stock          
Stockholders Equity Note [Line Items]          
Sale of common stock, shares 1,498,662        
Common Stock          
Stockholders Equity Note [Line Items]          
Amount of votes common stock is entitled One        
Percentage of Board entitled to elect 25.00%        
Treasury stock, shares 4,823,988 6,322,650      
Class B Common Stock          
Stockholders Equity Note [Line Items]          
Amount of votes common stock is entitled Ten        
Treasury stock, shares 48,263 48,263