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Stockholders' Equity - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Jul. 16, 2015
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Subsequent Event [Line Items]            
Stock repurchased and retired, total costs       $ 772,000 $ 597,000  
Dividends per common share   $ 0.30 $ 0.27 $ 0.60 $ 0.55  
Common Stock [Member]            
Subsequent Event [Line Items]            
Repurchase and retirement of common stock, shares       16,958 13,541  
Share Repurchase Plan [Member]            
Subsequent Event [Line Items]            
Common stock repurchase authorized amount           $ 40,000,000
Common stock repurchased and retired, weighted average price     $ 36.03 $ 42.49 $ 36.45  
Stock repurchased and retired, total costs     $ 10,005,000 $ 1,610,000 $ 17,810,000  
Fees and commissions average price repurchase common stock     $ 36.05 $ 42.51 $ 36.47  
Time through the company is allowed to repurchase shares       Mar. 31, 2015    
Share Repurchase Plan [Member] | Common Stock [Member]            
Subsequent Event [Line Items]            
Repurchase and retirement of common stock, shares   0 277,510 37,869 488,346  
Share Repurchase Plan [Member] | Subsequent Event [Member]            
Subsequent Event [Line Items]            
Dividends per common share $ 0.30          
Date of dividend payable Sep. 18, 2015          
Record date of dividend payable Aug. 21, 2015