Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 24,782	$ 24,322	$ 68,405	$ 67,309
Undistributed income	23,509	23,049	67,132	66,036
Allocation of net income:
Allocation of undistributed income	23,509	23,049	67,132	66,036
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	24,081	23,634	66,470	65,405
Dividends	(1,237)	(1,237)	(1,237)	(1,237)
Undistributed income	22,844	22,397	65,233	64,168
Allocation of net income:
Dividends	1,237	1,237	1,237	1,237
Allocation of undistributed income	22,844	22,397	65,233	64,168
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,435	3,435	3,435	3,435
Effect of dilutive stock options (in shares)	4	0	5	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,439	3,435	3,440	3,435
Basic Earnings Per Share (in dollars per share)	$ 7.01	$ 6.88	$ 19.35	$ 19.04
Diluted Earnings Per Share (in dollars per share)	$ 7.00	$ 6.88	$ 19.32	$ 19.04
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	701	688	1,935	1,904
Dividends	(36)	(36)	(36)	(36)
Undistributed income	665	652	1,899	1,868
Allocation of net income:
Dividends	36	36	36	36
Allocation of undistributed income	$ 665	$ 652	$ 1,899	$ 1,868
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)	$ 3.51	$ 3.44	$ 9.68	$ 9.52
Diluted Earnings Per Share (in dollars per share)	$ 3.51	$ 3.44	$ 9.68	$ 9.52