Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Allocation of net income:
Allocation of undistributed income	$ 23,049	$ 17,994	$ 66,036	$ 49,994
Net income	24,322	19,265	67,309	51,265
Class A [Member]
Allocation of net income:
Dividends	1,237	1,235	1,237	1,235
Allocation of undistributed income	22,397	17,485	64,168	48,580
Net income	23,634	18,720	65,405	49,815
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,435	3,435	3,435	3,433
Effect of dilutive stock options (in shares)	0	1	0	4
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,435	3,436	3,435	3,437
Basic Earnings Per Share (in dollars per share)	$ 6.88	$ 5.45	$ 19.04	$ 14.51
Diluted Earnings Per Share (in dollars per share)	$ 6.88	$ 5.45	$ 19.04	$ 14.49
Class B [Member]
Allocation of net income:
Dividends	36	36	36	36
Allocation of undistributed income	652	509	1,868	1,414
Net income	$ 688	$ 545	$ 1,904	$ 1,450
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)	$ 3.44	$ 2.73	$ 9.52	$ 7.25
Diluted Earnings Per Share (in dollars per share)	$ 3.44	$ 2.73	$ 9.52	$ 7.25