Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 28,466	$ 24,782	$ 77,475	$ 68,405
Undistributed income	27,194	23,509	76,203	67,132
Allocation of net income:
Allocation of undistributed income	27,194	23,509	76,203	67,132
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	27,661	24,081	75,284	66,470
Dividends	(1,236)	(1,237)	(1,236)	(1,237)
Undistributed income	26,425	22,844	74,048	65,233
Allocation of net income:
Dividends	1,236	1,237	1,236	1,237
Allocation of undistributed income	26,425	22,844	74,048	65,233
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,436	3,435	3,435	3,435
Effect of dilutive stock options (in shares)	2	4	2	5
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,438	3,439	3,437	3,440
Basic Earnings Per Share (in dollars per share)	$ 8.05	$ 7.01	$ 21.92	$ 19.35
Diluted Earnings Per Share (in dollars per share)	$ 8.05	$ 7.00	$ 21.91	$ 19.32
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	805	701	2,191	1,935
Dividends	(36)	(36)	(36)	(36)
Undistributed income	769	665	2,155	1,899
Allocation of net income:
Dividends	36	36	36	36
Allocation of undistributed income	$ 769	$ 665	$ 2,155	$ 1,899
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)	$ 4.03	$ 3.51	$ 10.95	$ 9.68
Diluted Earnings Per Share (in dollars per share)	$ 4.03	$ 3.51	$ 10.95	$ 9.68