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Earnings Per Share (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Weighted average number of shares for calculation of basic EPS – Common Stock 36,720,749 36,448,138 36,031,792
Stock options under equity incentive plans 92,970 152,665 443,310
Weighted average number of shares for calculation of diluted EPS – Common Stock 36,813,719 36,600,803 36,475,102
Stock options outstanding for equity incentive plans      
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Common stock that have been excluded from the diluted shares calculation 2,055,784 1,314,784 17,534