Share-Based Compensation Plans - Summary of Stock Activity under PIP (Detail) (Performance Incentive Plan [Member], USD $)	1 Months Ended	12 Months Ended
	Jul. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Performance Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Outstanding Beginning Balance		87,414	101,150
Number of Shares, Granted	4,800	35,706	41,664
Number of Shares, Vested		13,837	31,400
Number of Shares, Forfeited		21,600	24,000
Number of Shares, Expired		3,038
Number of Shares, Outstanding Ending Balance		84,645	87,414	101,150
Weighted Average Grant Date Fair Value, Outstanding Beginning Balance		$ 34.47	$ 28.78
Weighted Average Grant Date Fair Value, Granted		$ 39.62	$ 40.16	$ 29.38
Weighted Average Grant Date Fair Value, Vested		$ 29.19	$ 27.63
Weighted Average Grant Date Fair Value, Forfeited		$ 36.57	$ 29.31
Weighted Average Grant Date Fair Value, Expired		$ 26.29
Weighted Average Grant Date Fair Value, Outstanding Ending Balance		$ 37.86	$ 34.47	$ 28.78