Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	May 14, 2019	Jul. 02, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Dividends [Abstract]
Cash dividends declared per share of common stock (in dollars per share)			$ 7.05	$ 6.90	$ 7.05	$ 6.90
Basic Earnings per Common Share [Abstract]
Net Income			$ 13,738	$ 12,000	$ 41,392	$ 32,491
Weighted Average Number of Common Shares Outstanding (in shares)			787,307	793,418	786,361	807,129
Basic Earnings Per Common Share Amount (in dollars per share)			$ 17.45	$ 15.12	$ 52.64	$ 40.26
Mid-Year Cash Dividend Declared in 2019 [Member]
Dividends [Abstract]
Dividends payable, date declared					May 14, 2019
Cash dividends declared per share of common stock (in dollars per share)	$ 7.05
Percentage increase in cash dividend per share	2.20%
Dividends payable, date of record					Jun. 14, 2019
Dividends payable, date paid					Jul. 01, 2019
Mid-Year Cash Dividend Declared in 2018 [Member]
Dividends [Abstract]
Cash dividends paid per share of common stock (in dollars per share)		$ 6.90
Dividends payable, date paid					Jul. 02, 2018