Earnings Per Share (EPS Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	$ 10,825,000	$ 19,989,000	$ 57,176,000	$ 93,883,000
Dividends			0	0
Undistributed earnings	10,825,000	19,989,000	57,176,000	93,883,000
Allocation of net earnings:
Dividends			0	0
Undistributed earnings	10,825,000	19,989,000	57,176,000	93,883,000
Net earnings	10,825,000	19,989,000	57,176,000	93,883,000
Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	10,519,000	19,424,000	55,560,000	91,228,000
Dividends	0	0	0	0
Undistributed earnings	10,519,000	19,424,000	55,560,000	91,228,000
Allocation of net earnings:
Dividends	0	0	0	0
Undistributed earnings	10,519,000	19,424,000	55,560,000	91,228,000
Net earnings	$ 10,519,000	$ 19,424,000	$ 55,560,000	$ 91,228,000
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,436	3,436	3,436	3,436
Effect of dilutive stock options (in shares)	0	0	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,436	3,436	3,436	3,436
Basic earnings per share (in dollars per share)	$ 3.06	$ 5.65	$ 16.17	$ 26.55
Diluted earnings per share (in dollars per share)	$ 3.06	$ 5.65	$ 16.17	$ 26.55
Class B
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	$ 306,000	$ 565,000	$ 1,616,000	$ 2,655,000
Dividends	0	0	0	0
Undistributed earnings	306,000	565,000	1,616,000	2,655,000
Allocation of net earnings:
Dividends	0	0	0	0
Undistributed earnings	306,000	565,000	1,616,000	2,655,000
Net earnings	$ 306,000	$ 565,000	$ 1,616,000	$ 2,655,000
Denominator:
Basic earnings per share - weighted-average shares (in shares)	200	200	200	200
Effect of dilutive stock options (in shares)	0	0	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	200	200	200	200
Basic earnings per share (in dollars per share)	$ 1.53	$ 2.83	$ 8.08	$ 13.28
Diluted earnings per share (in dollars per share)	$ 1.53	$ 2.83	$ 8.08	$ 13.28