Stock-Based Compensation Plans (Summary Of Stock Option Activity) (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Common Shares
Common Shares, Outstanding at beginning of the period (in shares)	1,698,912	1,551,734
Common Shares, Granted (in shares)	0	562,166
Common Shares, Exercised (in shares)	(499,222)	(184,366)
Common Shares, Forfeited or canceled (in shares)	(16,378)	(86,039)
Common Shares, Outstanding at end of the period (in shares)	1,183,312	1,843,495
Stock Options, Exercisable (in shares)	640,759	813,666
Weighted Average Strike Price
Weighted Average Strike Price, Outstanding at beginning of period (usd per share)	$ 41.50	$ 41.32
Weighted Average Strike Price, Granted (usd per share)	0.00	41.04
Weighted Average Strike Price, Exercised (usd per share)	40.57	37.43
Weighted Average Strike Price, Forfeited or canceled (usd per share)	43.07	48.93
Weighted Average Strike Price, Outstanding at end of period (usd per share)	41.87	41.27
Stock Options, Weighted Average Strike Price, Exercisable (usd per share)	$ 41.58	$ 39.27
Stock Options, Remaining Contractual Term, Outstanding, Years	4 years 2 months 12 days	4 years 9 months 18 days
Stock Options, Remaining Contractual Term, Exercisable, Years	3 years 6 months	3 years 6 months
Stock Options, Intrinsic Value, Outstanding	$ 43,122	$ 26,363
Stock Options, Intrinsic Value, Exercisable	$ 23,532	$ 13,265