Dividends and Basic Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended		9 Months Ended
	Jul. 01, 2017	May 09, 2017	Jul. 01, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Dividends Payable [Line Items]
Cash dividends declared per share of common stock (in dollars per share)						$ 6.75	$ 6.55
Earnings per share for the period [Abstract]
Net Income				$ 8,581	$ 7,538	$ 24,589	$ 22,042
Weighted Average Number of Common Shares Outstanding (in shares)				810,291	792,387	809,002	792,223
Basic Earnings Per Common Share Amount (in dollars per share)				$ 10.59	$ 9.51	$ 30.39	$ 27.82
Mid-Year Cash Dividend Declared in 2017 [Member]
Dividends Payable [Line Items]
Dividends payable, date declared						May 09, 2017
Cash dividends declared per share of common stock (in dollars per share)		$ 6.75
Percentage increase in cash dividend per share		3.10%
Cash dividends paid per share of common stock (in dollars per share)	$ 6.75
Dividends payable, date paid	Jul. 01, 2017
Dividends payable, date of record						Jun. 09, 2017
Mid-Year Cash Dividend Declared in 2016 [Member]
Dividends Payable [Line Items]
Cash dividends paid per share of common stock (in dollars per share)			$ 6.55
Dividends payable, date paid						Jul. 01, 2016