Capital Stock - Common Stock and Treasury Stock Activity (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Common shares
Balance at the beginning of the year	153,412,050	159,906,032
Stock plan shares issued	2,068,746	2,518,071
Shares surrendered by optionees	(180,537)	(204,538)
Common stock repurchases	(3,393,462)	(8,807,515)
Balance at the end of the year	151,906,797	153,412,050
Treasury shares
Balance at the beginning of the year	24,305,930	15,293,877
Stock plan shares issued	0	0
Shares surrendered by optionees	180,537	204,538
Common stock repurchases	3,393,462	8,807,515
Balance at the end of the year	27,879,929	24,305,930