Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Allocation of net income:
Allocation of undistributed income	$ 23,246	$ 13,790	$ 42,987	$ 32,001
Net income	23,246	13,790	42,987	32,001
Class A [Member]
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	22,589	13,400	41,771	31,096
Net income	22,589	13,400	41,771	31,096
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,435	3,434	3,435	3,432
Effect of dilutive stock options (in shares)	0	3	0	6
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,435	3,437	3,435	3,438
Basic Earnings Per Share (in dollars per share)	$ 6.58	$ 3.90	$ 12.16	$ 9.06
Diluted Earnings Per Share (in dollars per share)	$ 6.58	$ 3.90	$ 12.16	$ 9.04
Class B [Member]
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	658	390	1,216	905
Net income	$ 658	$ 390	$ 1,216	$ 905
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)	$ 3.29	$ 1.95	$ 6.08	$ 4.53
Diluted Earnings Per Share (in dollars per share)	$ 3.29	$ 1.95	$ 6.08	$ 4.53