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Employee Share Plans - Schedule of Share-Based Compensation Awards (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]      
Stock options $ 2,105 $ 2,253 $ 2,541
Restricted stock awards 3,600 3,472 3,188
Total share-based compensation expense before income tax benefit $ 5,705 $ 5,725 $ 5,729