Share-Based Compensation Plans - Summary of Information Relative to Stock Option Plan Activity (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options Outstanding, Beginning of Period	230,760
Number of Options, Granted	7,641
Number of Options, Exercised	(44,177)
Number of Options, Forfeited	(28,283)
Number of Options Outstanding, End of Period	165,941	230,760
Number of Options Exercisable	149,407
Weighted Average Exercise Price, Options Outstanding, Beginning of Period	$ 39.51
Weighted Average Exercise Price, Granted	$ 39.51	$ 36.47	$ 34.30
Weighted Average Exercise Price, Exercised	$ 29.64
Weighted Average Exercise Price, Forfeited	$ 55.20
Weighted Average Exercise Price, Options Outstanding, End of Period	$ 39.46	$ 39.51
Weighted Average Exercise Price Exercisable	$ 39.64
Weighted Average Remaining Contractual Term, Outstanding	3 years 10 months 24 days
Weighted Average Remaining Contractual Term, Exercisable	3 years 5 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 762
Aggregate Intrinsic Value, Exercisable	$ 700