Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends and Basic Earnings Per Common Share [Abstract]
Cash dividends									$ 10,982	$ 10,478	$ 10,157
Cash dividends paid per share of common stock (in dollars per share)									$ 13.55	$ 13.10	$ 12.90
Percentage increase in cash dividend per share									3.40%
Earnings per share for the period [Abstract]
Net Income	$ 3,781	$ 8,581	$ 8,187	$ 7,821	$ 7,681	$ 7,538	$ 7,332	$ 7,172	$ 28,370	$ 29,723	$ 27,392
Weighted Average Number of Common Shares Outstanding (in shares)									809,834	793,970	786,582
Basic Earnings Per Common Share (in dollars per share)	$ 4.64	$ 10.59	$ 10.12	$ 9.68	$ 9.62	$ 9.51	$ 9.25	$ 9.06	$ 35.03	$ 37.44	$ 34.82