Stock-Based Compensation Plans (Summary Of Plans' Restricted Share And Performance-Vested Stock Award Activity) (Detail) - $ / shares	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock
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	137,593	146,112
Shares, Granted	10,516	12,300
Shares, Vested and issued	(3,726)	(4,925)
Shares, Outstanding, End of the Period	144,383	153,487
Shares, Vested, but not issuable	88,493	85,000
Weighted Average Grant-Date Fair Value	$ 49.14	$ 47.53	$ 49.63	$ 47.45
Weighted Average Grant-Date Fair Value, Granted	40.77	44.11
Weighted Average Grant-Date Fair Value, Vested and issued	43.87	36.74
Weighted Average Grant-Date Fair Value, Vested, but not issuable	$ 51.43	$ 51.88
Performance Shares
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	276,533	295,679
Shares, Granted	116,576	102,828
Shares, Vested and issued	(78,410)	(78,590)
Shares, Forfeited	(7,417)	(29,926)
Shares, Outstanding, End of the Period	307,282	289,991
Shares, Vested, but not issuable	6,612	0
Weighted Average Grant-Date Fair Value	$ 43.59	$ 42.90	$ 43.01	$ 38.18
Weighted Average Grant-Date Fair Value, Granted	40.87	44.11
Weighted Average Grant-Date Fair Value, Vested and issued	37.90	31.10
Weighted Average Grant-Date Fair Value, Forfeited	39.32	31.41
Weighted Average Grant-Date Fair Value, Vested, but not issuable	$ 37.85	$ 0.00