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Employee Share Plans - Schedule of Share-Based Compensation Awards (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Compensation And Retirement Disclosure [Abstract]        
Stock options $ 527 $ 574 $ 1,578 $ 1,717
Restricted stock awards 930 916 2,669 2,615
Total stock-based compensation expense $ 1,457 $ 1,490 $ 4,247 $ 4,332