Dividends and Basic Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dividends and Basic Earnings Per Share [Abstract]
Cash dividends	$ 9,418	$ 9,158	$ 8,855
Cash dividends per share of common stock (in dollars per share)	$ 12.10	$ 11.75	$ 11.35
Percentage increase in cash dividend per share (in hundredths)	2.80%
Earnings per share for the period [Abstract]
Net Income	$ 23,349	$ 22,209	$ 21,118
Average Number of Common Shares Outstanding (in shares)	778,648	779,424	780,619
Basic Earnings Per Common Share (in dollars per share)	$ 29.99	$ 28.49	$ 27.05