Earnings Per Share (EPS Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 19,989	$ 35,641	$ 93,883	$ 94,982
Undistributed earnings	19,989	35,641	93,883	94,982
Allocation of net income:
Undistributed earnings	19,989	35,641	93,883	94,982
Net earnings	19,989	35,641	93,883	94,982
Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	19,424	34,633	91,228	92,296
Dividends	0	0	0	0
Undistributed earnings	19,424	34,633	91,228	92,296
Allocation of net income:
Dividends	0	0	0	0
Undistributed earnings	19,424	34,633	91,228	92,296
Net earnings	$ 19,424	$ 34,633	$ 91,228	$ 92,296
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)	$ 5.65	$ 10.08	$ 26.55	$ 26.86
Diluted Earnings Per Share (in dollars per share)	$ 5.65	$ 10.08	$ 26.55	$ 26.86
Class B
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 565	$ 1,008	$ 2,655	$ 2,686
Dividends	0	0	0	0
Undistributed earnings	565	1,008	2,655	2,686
Allocation of net income:
Dividends	0	0	0	0
Undistributed earnings	565	1,008	2,655	2,686
Net earnings	$ 565	$ 1,008	$ 2,655	$ 2,686
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)	$ 2.83	$ 5.04	$ 13.28	$ 13.43
Diluted Earnings Per Share (in dollars per share)	$ 2.83	$ 5.04	$ 13.28	$ 13.43