Stock-Based Compensation Plans (Summary Of Stock Option Activity) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Common Shares
Common Shares, Outstanding at beginning of the period (in shares)	520,663	755,332
Common Shares, Granted (in shares)	0	0
Common Shares, Exercised (in shares)	(208,579)	(94,653)
Common Shares, Forfeited or canceled (in shares)	(590)	0
Common Shares, Outstanding at end of the period (in shares)	311,494	660,679
Common Shares, Exercisable (in shares)	309,945	650,456
Weighted Average Strike Price
Weighted Average Strike Price, Outstanding at beginning of period (usd per share)	$ 42.47	$ 42.43
Weighted Average Strike Price, Granted (usd per share)	0	0
Weighted Average Strike Price, Exercised (usd per share)	42.96	38.69
Weighted Average Strike Price, Forfeited or canceled (usd per share)	46.86	0
Weighted Average Strike Price, Outstanding at end of period (usd per share)	42.14	42.97
Weighted Average Strike Price, Exercisable (usd per share)	$ 42.11	$ 42.95
Remaining Contractual Term, Outstanding	1 year 9 months 18 days	2 years 6 months
Remaining Contractual Term, Exercisable	1 year 9 months 18 days	2 years 4 months 24 days
Intrinsic Value, Outstanding	$ 10,485	$ 0
Intrinsic Value, Exercisable	$ 10,441	$ 1

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.