Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividends and Basic Earnings Per Common Share [Abstract]
Cash dividends declared on common stock									$ 11,221	$ 11,151	$ 10,982
Cash dividends paid per share of common stock (in dollars per share)									$ 14.20	$ 13.90	$ 13.55
Percentage increase in cash dividend per share									2.20%
Basic Earnings per Common Share [Abstract]
Net Income	$ 14,644	$ 13,738	$ 14,105	$ 13,549	$ 13,036	$ 12,000	$ 10,550	$ 9,941	$ 56,036	$ 45,527	$ 28,370
Weighted Average Number of Common Shares Outstanding (in shares)									787,227	801,229	809,834
Basic Earnings Per Common Share (in dollars per share)									$ 71.18	$ 56.82	$ 35.03