Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
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	336,709	299,040
Shares, Granted	109,557	90,285
Shares, Vested and issued	(123,629)	(37,651)
Shares, Forfeited	(1,353)	(2,000)
Shares, Outstanding, End of the Period	321,284	349,674
Shares, Vested, but not issuable	85,320	85,000
Weighted Average Grant-Date Fair Value	$ 37.76	$ 38.58	$ 38.29	$ 39.44
Weighted Average Grant-Date Fair Value, Granted	$ 32.31	$ 33.16
Weighted Average Grant-Date Fair Value, Vested and issued	$ 34.46	$ 32.71
Weighted Average Grant-Date Fair Value, Forfeited	$ 30.99	$ 33.53
Weighted Average Grant-Date Fair Value, Vested, but not issuable	$ 51.80	$ 51.88
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	72,158	0
Shares, Granted	119,476	100,993
Shares, Vested and issued	0	0
Net change due to estimated performance	19,651	0
Shares, Forfeited	(3,897)	0
Shares, Outstanding, End of the Period	207,388	100,993
Weighted Average Grant-Date Fair Value	$ 31.78	$ 33.28	$ 33.25	$ 0.00
Weighted Average Grant-Date Fair Value, Granted	$ 31.10	$ 33.28
Weighted Average Grant-Date Fair Value, Vested and issued	$ 0.00	$ 0.00
Weighted Average Grant-Date Fair Value, Net change due to estimated performance	$ 30.55	$ 0.00
Weighted Average Grant-Date Fair Value, Forfeited	$ 32.07	$ 0.00