Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Shares, Beginning of the Period	314,226	336,709
Shares, Granted	10,617	109,557
Shares, Vested and issued	(135,767)	(123,629)
Shares, Forfeited	(1,236)	(1,353)
Shares, Outstanding, End of the Period	187,840	321,284
Shares, Vested, but not issuable	85,000	85,320
Weighted Average Grant-Date Fair Value	$ 42.51	$ 37.76	$ 37.99	$ 38.29
Weighted Average Grant-Date Fair Value, Granted	$ 40.86	$ 32.31
Weighted Average Grant-Date Fair Value, Vested and issued	$ 31.97	$ 34.46
Weighted Average Grant-Date Fair Value, Forfeited	$ 35.02	$ 30.99
Weighted Average Grant-Date Fair Value, Vested, but not issuable	$ 51.88	$ 51.80
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Shares, Beginning of the Period	153,915	72,158
Shares, Granted	105,825	119,476
Shares, Vested and issued	0	0
Net change due to estimated performance	21,249	(19,651)
Shares, Forfeited	(6,115)	(3,897)
Shares, Outstanding, End of the Period	232,376	207,388
Weighted Average Grant-Date Fair Value	$ 34.10	$ 31.78	$ 31.78	$ 33.25
Weighted Average Grant-Date Fair Value, Granted	$ 37.87	$ 31.10
Weighted Average Grant-Date Fair Value, Vested and issued	$ 0.00	$ 0.00
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options, Net Decrease Due To Estimated Performance In Period Weighted Average Grant Date Fair Value	$ 36.05	$ 30.55
Weighted Average Grant-Date Fair Value, Forfeited	$ 34.29	$ 32.07