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Employee Share Plans - Additional Information (Detail)
shares in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
$ / shares
shares
Jun. 30, 2017
shares
Compensation And Retirement Disclosure [Abstract]    
Maximum stock based compensation awards granted under the plan | shares 9.6 9.6
Stock awards expiry   2024
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Method Used   Black-Scholes-Merton options-pricing model
Fair value of stock options granted | $ / shares $ 3.49