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Capital Stock - Common Stock and Treasury Stock Activity (Detail) - shares
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Common shares    
Balance at the beginning of the year 139,555,487 140,498,981
Stock plan shares issued 2,175,510 1,281,198
Shares surrendered by optionees (159,089) (185,141)
Common stock repurchases (2,911,754) (2,039,551)
Balance at the end of the year 138,660,154 139,555,487
Treasury shares    
Balance at the beginning of the year 42,335,315 40,110,623
Shares surrendered by optionees 159,089 185,141
Common stock repurchases 2,911,754 2,039,551
Balance at the end of the year 45,406,158 42,335,315