Stock Options Outstanding (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Number of Shares
Beginning balance	105,250	157,500	191,000
Granted
Exercised	(15,000)	(52,250)	(29,000)
Expired or forfeited			(4,500)
Ending balance	90,250	105,250	157,500
Number of options exercisable	90,250	105,250	157,500
Average Exercise Price
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Beginning of Period	$ 29.50	$ 27.13	$ 26.21
Granted
Exercised	$ 26.90	$ 22.35	$ 21.12
Expired or forfeited			$ 26.69
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, End of Period	$ 29.94	$ 29.50	$ 27.13
Number of options exercisable	$ 29.94	$ 29.50	$ 27.13