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Capital Stock (Detail) - shares
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Class Of Stock [Line Items]    
Beginning balance 248,129,430 247,144,356
Restricted stock grants 804,768 539,096
Treasury shares issued 4,900 15,580
Ending balance 249,427,206 248,129,430
Restricted Stock    
Class Of Stock [Line Items]    
Restricted stock units vested 411,959 344,937
Performance Share    
Class Of Stock [Line Items]    
Performance stock units issued 76,149 85,461