Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Dividends and Basic Earnings Per Common Share [Abstract]
Cash dividends	$ 10,157	$ 9,919	$ 9,723
Cash dividends per share of common stock (in dollars per share)	$ 12.90	$ 12.70	$ 12.50
Percentage increase in cash dividend per share	1.60%
Earnings per share for the period [Abstract]
Net Income	$ 27,392	$ 25,402	$ 24,061
Weighted Average Number of Common Shares Outstanding (in shares)	786,582	778,358	777,882
Basic Earnings Per Common Share (in dollars per share)	$ 34.82	$ 32.64	$ 30.93