Dividends and Basic and Diluted Earnings Per Common Share (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividends and Basic and Diluted Earnings Per Common Share [Abstract]
Cash dividends declared on common stock									$ 11,700,000	$ 11,221,000	$ 11,151,000
Cash dividends paid per share of common stock (in dollars per share)									$ 14.75	$ 14.20	$ 13.90
Percentage increase in cash dividend per share									3.90%
Basic and Diluted Earnings per Common Share [Abstract]
Net Income	$ 15,493,000	$ 14,810,000	$ 14,309,000	$ 14,122,000	$ 14,644,000	$ 13,738,000	$ 14,105,000	$ 13,549,000	$ 58,734,000	$ 56,036,000	$ 45,527,000
Weighted Average Number of Common Shares Outstanding (in shares)									793,337	787,227	801,229
Basic and Diluted Earnings Per Common Share (in dollars per share)	$ 19.54	$ 18.66	$ 18.03	$ 17.80	$ 18.54	$ 17.45	$ 17.92	$ 17.27	$ 74.03	$ 71.18	$ 56.82