Annual report pursuant to Section 13 and 15(d)

Share-based Compensation - Summary of Stock Options Outstanding under Plans (Detail)

v3.8.0.1
Share-based Compensation - Summary of Stock Options Outstanding under Plans (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Options Outstanding, Number of Shares 364,088 387,901 510,148 603,872
Options Outstanding, Intrinsic Value $ 4,548      
Options Outstanding, Weighted-Average Exercise Price Per Share (in usd per share) $ 17.86 $ 17.49 $ 24.38 $ 23.78
Options Outstanding, Weighted-Average Remaining Contractual Life      
Options Exercisable, Number of Shares 354,753 341,561 333,460  
Options Exercisable, Intrinsic Value $ 4,483      
Options Exercisable, Weighted-Average Exercise Price Per Share (in usd per share) $ 17.71 $ 16.85 $ 27.16  
Options Exercisable, Weighted-Average Remaining Contractual Life      
Intrinsic value of stock options outstanding (in usd per share) $ 30.35      
$10.88 to $14.99 [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise Price Range, Lower Limit (in usd per share) 10.88      
Exercise Price Range, Upper Limit (in usd per share) $ 14.99      
Options Outstanding, Number of Shares 13,088      
Options Outstanding, Intrinsic Value $ 235      
Options Outstanding, Weighted-Average Exercise Price Per Share (in usd per share) $ 12.37      
Options Outstanding, Weighted-Average Remaining Contractual Life 4 years 7 months      
Options Exercisable, Number of Shares 13,088      
Options Exercisable, Intrinsic Value $ 235      
Options Exercisable, Weighted-Average Exercise Price Per Share (in usd per share) $ 12.37      
Options Exercisable, Weighted-Average Remaining Contractual Life 4 years 7 months      
$15.00 to $19.99 [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise Price Range, Lower Limit (in usd per share) $ 15.00      
Exercise Price Range, Upper Limit (in usd per share) $ 19.99      
Options Outstanding, Number of Shares 255,000      
Options Outstanding, Intrinsic Value $ 3,550      
Options Outstanding, Weighted-Average Exercise Price Per Share (in usd per share) $ 16.43      
Options Outstanding, Weighted-Average Remaining Contractual Life 5 years 8 months      
Options Exercisable, Number of Shares 255,000      
Options Exercisable, Intrinsic Value $ 3,550      
Options Exercisable, Weighted-Average Exercise Price Per Share (in usd per share) $ 16.43      
Options Exercisable, Weighted-Average Remaining Contractual Life 5 years 8 months      
$20.00 to $24.83 [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Exercise Price Range, Lower Limit (in usd per share) $ 20.00      
Exercise Price Range, Upper Limit (in usd per share) $ 24.83      
Options Outstanding, Number of Shares 96,000      
Options Outstanding, Intrinsic Value $ 763      
Options Outstanding, Weighted-Average Exercise Price Per Share (in usd per share) $ 22.41      
Options Outstanding, Weighted-Average Remaining Contractual Life 6 years 8 months      
Options Exercisable, Number of Shares 86,665      
Options Exercisable, Intrinsic Value $ 698      
Options Exercisable, Weighted-Average Exercise Price Per Share (in usd per share) $ 22.29      
Options Exercisable, Weighted-Average Remaining Contractual Life 6 years 7 months