Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	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
Numerator for Basic and Diluted Earnings Per Share:
Net earnings	$ 35,135	$ 10,825	$ 48,413	$ (2,062)	$ 37,733	$ 19,989	$ 33,696	$ 40,198	$ 92,312	$ 131,616	$ 116,758
Undistributed earnings									91,039	130,343	115,485
Allocation of net earnings:
Undistributed earnings									91,039	130,343	115,485
Net earnings	$ 35,135	$ 10,825	$ 48,413	$ (2,062)	$ 37,733	$ 19,989	$ 33,696	$ 40,198	92,312	131,616	116,758
Class A
Numerator for Basic and Diluted Earnings Per Share:
Net earnings									89,701	127,894	113,456
Dividends									(1,237)	(1,237)	(1,237)
Undistributed earnings									88,464	126,657	112,219
Allocation of net earnings:
Dividends									1,237	1,237	1,237
Undistributed earnings									88,464	126,657	112,219
Net earnings									$ 89,701	$ 127,894	$ 113,456
Denominator:
Basic earnings per share - weighted-average shares (in shares)									3,436	3,436	3,436
Effect of dilutive stock options (in shares)									0	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									3,436	3,436	3,436
Basic earnings per share (in dollars per share)	$ 9.94	$ 3.06	$ 13.69	$ (0.58)	$ 10.67	$ 5.65	$ 9.53	$ 11.37	$ 26.11	$ 37.22	$ 33.02
Diluted earnings per share (in dollars per share)	9.94	3.06	13.69	(0.58)	10.67	5.65	9.53	11.37	$ 26.11	$ 37.22	$ 33.02
Class B
Numerator for Basic and Diluted Earnings Per Share:
Net earnings									$ 2,611	$ 3,722	$ 3,302
Dividends									(36)	(36)	(36)
Undistributed earnings									2,575	3,686	3,266
Allocation of net earnings:
Dividends									36	36	36
Undistributed earnings									2,575	3,686	3,266
Net earnings									$ 2,611	$ 3,722	$ 3,302
Denominator:
Basic earnings per share - weighted-average shares (in shares)									200	200	200
Effect of dilutive stock options (in shares)									0	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									200	200	200
Basic earnings per share (in dollars per share)	4.97	1.53	6.85	(0.29)	5.34	2.83	4.76	5.68	$ 13.05	$ 18.61	$ 16.51
Diluted earnings per share (in dollars per share)	$ 4.97	$ 1.53	$ 6.85	$ (0.29)	$ 5.34	$ 2.83	$ 4.76	$ 5.68	$ 13.05	$ 18.61	$ 16.51