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Shareholders' Equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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 $ 114,425,000 $ 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 4,823,988      
Class B Common Stock          
Stockholders Equity Note [Line Items]          
Amount of votes common stock is entitled   Ten      
Treasury stock, shares 48,263 48,263