Stock-Based Compensation Plans (Summary Of Stock Option Activity) (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Common Shares
Common Shares, Outstanding at beginning of the period (in shares)	1,084,756	1,698,912
Common Shares, Granted (in shares)	0	0
Common Shares, Exercised (in shares)	(271,674)	(499,222)
Common Shares, Forfeited or canceled (in shares)	(6,942)	(16,378)
Common Shares, Outstanding at end of the period (in shares)	806,140	1,183,312
Stock Options, Exercisable (in shares)	609,752	640,759
Weighted Average Strike Price
Weighted Average Strike Price, Outstanding at beginning of period (usd per share)	$ 41.98	$ 41.50
Weighted Average Strike Price, Granted (usd per share)	0.00	0.00
Weighted Average Strike Price, Exercised (usd per share)	41.36	40.57
Weighted Average Strike Price, Forfeited or canceled (usd per share)	39.81	43.07
Weighted Average Strike Price, Outstanding at end of period (usd per share)	42.20	41.87
Stock Options, Weighted Average Strike Price, Exercisable (usd per share)	$ 42.42	$ 41.58
Stock Options, Remaining Contractual Term, Outstanding, Years	3 years 3 months 18 days	4 years 2 months 12 days
Stock Options, Remaining Contractual Term, Exercisable, Years	3 years	3 years 6 months
Stock Options, Intrinsic Value, Outstanding	$ 34,452	$ 43,122
Stock Options, Intrinsic Value, Exercisable	$ 25,924	$ 23,532