Stock-Based Compensation Plans (Summary Of Stock Option Activity) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		27 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019
Common Shares
Common Shares, Outstanding at beginning of the period (in shares)	795,014	1,084,756
Common Shares, Granted (in shares)	0	0	0
Common Shares, Exercised (in shares)	(78,667)	(169,387)
Common Shares, Forfeited or canceled (in shares)	0	(1,703)
Common Shares, Outstanding at end of the period (in shares)	716,347	913,666	716,347
Common Shares, Exercisable (in shares)	701,227	712,535	701,227
Weighted Average Strike Price
Weighted Average Strike Price, Outstanding at beginning of period (usd per share)	$ 42.25	$ 41.98
Weighted Average Strike Price, Granted (usd per share)	0.00	0.00
Weighted Average Strike Price, Exercised (usd per share)	37.41	42.47
Weighted Average Strike Price, Forfeited or canceled (usd per share)	0.00	40.87
Weighted Average Strike Price, Outstanding at end of period (usd per share)	42.79	41.89	$ 42.79
Weighted Average Strike Price, Exercisable (usd per share)	$ 42.75	$ 42.00	$ 42.75
Remaining Contractual Term, Outstanding	2 years 10 months 24 days	3 years 8 months 12 days
Remaining Contractual Term, Exercisable	2 years 10 months 24 days	3 years 4 months 24 days
Intrinsic Value, Outstanding	$ 17,583	$ 40,351	$ 17,583
Intrinsic Value, Exercisable	$ 17,234	$ 31,391	$ 17,234