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Capital Stock (Detail) - shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Class Of Stock [Line Items]    
Beginning balance 249,510,022 248,129,430
Restricted stock grants 981,905 865,095
Treasury shares issued   5,302
Ending balance 251,138,134 249,510,022
Restricted Stock    
Class Of Stock [Line Items]    
Restricted stock units vested 633,460 434,046
Performance Share    
Class Of Stock [Line Items]    
Performance stock units issued 12,283 73,985
Performance stock dividends 464 2,164