Stock Option Accounting - Summary of Stock Option Activity under Stock Option Plan (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Shares under option:
Outstanding at beginning of year, Amount	23,350
Exercised, Amount	(1,625)	(5,987)
Forfeited, Amount	(1,350)
Outstanding at end of period, Amount	20,375
Exercisable at end of period, Amount	20,375
Available to be granted at end of period, Amount	224,884
Outstanding at beginning of year, Weighted Average Exercise Price	$ 31.17
Exercised, Weighted Average Exercise Price	$ 26.76
Forfeited, Weighted Average Exercise Price	$ 26.68
Outstanding at end of period, Weighted Average Exercise Price	$ 31.82
Exercisable at end of period, Weighted Average Exercise Price	$ 31.82