Stock-Based Compensation Plans (Summary of Plans' Restricted and Performance Share Award Activity) (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Restricted shares
Common Shares
Outstanding, beginning of the period (in shares)	610,155	476,813
Granted (in shares)	242,576	194,849
Vested and issued (in shares)	(96,649)	(52,465)
Expired, canceled or forfeited (in shares)	(1,968)	(3,615)
Outstanding, end of the period (in shares)	754,114	615,582
Vested, but deferred (in shares)	97,888	95,806
Weighted Average Grant-Date Fair Value
Outstanding, beginning of period (usd per share)	$ 73.21	$ 61.33
Granted (usd per share)	89.82	97.30
Vested and issued (usd per share)	64.14	65.42
Canceled or forfeited (usd per share)	77.37	66.37
Outstanding, end of period (usd per share)	79.70	72.34
Vested, but deferred (usd per share)	$ 53.30	$ 52.67
Performance-based stock
Common Shares
Outstanding, beginning of the period (in shares)	545,379	557,255
Granted (in shares)	185,514	159,202
Added by performance factor at vesting (in shares)	23,161	0
Vested and issued (in shares)	(178,203)	0
Expired, canceled or forfeited (in shares)	(2,301)	(159,952)
Outstanding, end of the period (in shares)	573,550	556,505
Vested, but deferred (in shares)	35,852	35,285
Weighted Average Grant-Date Fair Value
Outstanding, beginning of period (usd per share)	$ 70.30	$ 62.94
Granted (usd per share)	92.48	97.21
Added by performance factor at vesting (usd per share)	63.64	0
Vested and issued (usd per share)	63.64	0
Canceled or forfeited (usd per share)	78.20	71.56
Outstanding, end of period (usd per share)	79.24	70.27
Vested, but deferred (usd per share)	$ 44.59	$ 43.88