Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
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	102,160	116,939
Shares, Vested and issued	0	0
Net change due to estimated performance	0	0
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	$ 0.00	$ 0.00
Shares, Forfeited	(1,785)	(3,481)
Shares, Outstanding, End of the Period	254,290	185,616
Weighted Average Grant-Date Fair Value	$ 34.21	$ 31.85	$ 31.78	$ 33.25
Weighted Average Grant-Date Fair Value, Granted	$ 37.84	$ 30.98
Weighted Average Grant-Date Fair Value, Vested and issued	$ 0.00	$ 0.00
Weighted Average Grant-Date Fair Value, Forfeited	$ 33.07	$ 31.91
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	0	84,851
Shares, Vested and issued	(109,725)	(93,825)
Shares, Forfeited	(674)	(959)
Shares, Outstanding, End of the Period	203,827	326,776
Shares, Vested, but not issuable	85,000	85,000
Weighted Average Grant-Date Fair Value	$ 41.40	$ 37.38	$ 37.99	$ 38.29
Weighted Average Grant-Date Fair Value, Granted	$ 0.00	$ 30.98
Weighted Average Grant-Date Fair Value, Vested and issued	$ 31.67	$ 34.94
Weighted Average Grant-Date Fair Value, Forfeited	$ 32.10	$ 30.98
Weighted Average Grant-Date Fair Value, Vested, but not issuable	$ 51.88	$ 51.88