Stock Option Plan - Summary of option activity under Plan (Details) (Stock Option, Incentive Stock Option Plan, USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Stock Option | Incentive Stock Option Plan
Shares
Outstanding, beginning of year	32,400
Granted
Exercised	(10,400)
Forfeited or expired
Outstanding, end of year	22,000	32,400
Exercisable, end of year	19,000
Weighted-Average Exercise Price
Outstanding, beginning of year	$ 15.41
Exercised	$ 13.5
Outstanding, end of year	$ 16.31	$ 15.41
Weighted-average exercise price exercisable, end of year	$ 16.58
Weighted-Average Remaining Contractual Term
Weighted-average remaining contractual term	4 years 8 months 16 days	4 years 5 months 19 days
Weighted-average remaining contractual term exercisable, end of year	4 years 6 months 18 days
Aggregate Intrinsic Value
Outstanding, beginning of year	$ 37
Exercised	26
Outstanding, end of year	53	37
Exercisable, end of year	$ 43