Capital Stock and Equity Compensation (Summary of Activity Under Stock Option Plans) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Outstanding
Options outstanding at beginning of period (in shares)	33,283	116,575	212,038
Options exercised (in shares)	(22,333)	(83,292)	(95,113)
Options forfeited (in shares)	0	0	(350)
Options outstanding at end of period (in shares)	10,950	33,283	116,575
Weighted- Average Exercise Price Per Share
Weighted Average Exercise Price Per Share, Outstanding at period start (in dollars per share)	$ 36.40	$ 37.76	$ 40.47
Weighted Average Exercise Price Per Share, Options exercised (in dollars per share)	38.57	37.04	43.56
Weighted Average Exercise Price Per Share, Options forfeited (in dollars per share)	0.00	0.00	44.32
Weighted Average Exercise Price Per Share, Outstanding at period end (in dollars per share)	$ 31.99	$ 36.40	$ 37.76