Earnings Per Share (EPS Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	$ 48,413	$ 33,696	$ 46,351	$ 73,894
Dividends			0	0
Undistributed earnings	48,413	33,696	46,351	73,894
Allocation of net earnings:
Dividends			0	0
Undistributed earnings	48,413	33,696	46,351	73,894
Net earnings	48,413	33,696	46,351	73,894
Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	47,044	32,743	45,040	71,804
Dividends	0	0	0	0
Undistributed earnings	47,044	32,743	45,040	71,804
Allocation of net earnings:
Dividends	0	0	0	0
Undistributed earnings	47,044	32,743	45,040	71,804
Net earnings	$ 47,044	$ 32,743	$ 45,040	$ 71,804
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)	$ 13.69	$ 9.53	$ 13.11	$ 20.90
Diluted earnings per share (in dollars per share)	$ 13.69	$ 9.53	$ 13.11	$ 20.90
Class B
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings	$ 1,369	$ 953	$ 1,311	$ 2,090
Dividends	0	0	0	0
Undistributed earnings	1,369	953	1,311	2,090
Allocation of net earnings:
Dividends	0	0	0	0
Undistributed earnings	1,369	953	1,311	2,090
Net earnings	$ 1,369	$ 953	$ 1,311	$ 2,090
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)	$ 6.85	$ 4.76	$ 6.55	$ 10.45
Diluted earnings per share (in dollars per share)	$ 6.85	$ 4.76	$ 6.55	$ 10.45