Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allocation of net income:
Allocation of undistributed income									$ 91,285	$ 54,354	$ 71,627
Net income	25,249	24,322	23,246	19,741	4,362	19,265	13,790	18,211	92,558	55,627	72,897
Class A [Member]
Allocation of net income:
Dividends									(1,237)	(1,237)	(1,234)
Allocation of undistributed income									88,703	52,816	69,598
Net income									89,940	54,053	70,832
Denominator:
Basic earnings per share - weighted-average shares (in shares)									3,435	3,433	3,427
Effect of dilutive stock options (in shares)									0	4	10
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									3,435	3,437	3,437
Basic Earnings Per Share (in dollars per share)	$ 7.14	$ 6.88	$ 6.58	$ 5.58	$ 1.23	$ 5.45	$ 3.90	$ 5.16	$ 26.19	$ 15.74	$ 20.67
Diluted Earnings Per Share (in dollars per share)	$ 7.14	$ 6.88	$ 6.58	$ 5.58	$ 1.23	$ 5.45	$ 3.90	$ 5.15	$ 26.19	$ 15.73	$ 20.61
Class B [Member]
Allocation of net income:
Dividends									(36)	(36)	(36)
Allocation of undistributed income									2,582	1,538	2,029
Net income									$ 2,618	$ 1,574	$ 2,065
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)	$ 3.57	$ 3.44	$ 3.29	$ 2.79	$ 0.62	$ 2.73	$ 1.95	$ 2.58	$ 13.09	$ 7.87	$ 10.33
Diluted Earnings Per Share (in dollars per share)	$ 3.57	$ 3.44	$ 3.29	$ 2.79	$ 0.62	$ 2.73	$ 1.95	$ 2.58	$ 13.09	$ 7.87	$ 10.33