Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Dividends and Basic Earnings Per Common Share [Abstract]
Cash dividends	$ 10,478	$ 10,157	$ 9,919
Cash dividends paid per share of common stock (in dollars per share)	$ 13.10	$ 12.90	$ 12.70
Percentage increase in cash dividend per share	1.60%
Earnings per share for the period [Abstract]
Net Income	$ 29,723	$ 27,392	$ 25,402
Weighted Average Number of Common Shares Outstanding (in shares)	793,970	786,582	778,358
Basic Earnings Per Common Share (in dollars per share)	$ 37.44	$ 34.82	$ 32.64