Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 16,632	$ 19,741
Allocation of net income:
Allocation of undistributed income	16,632	19,741
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	16,162	19,183
Allocation of net income:
Dividends	0	0
Allocation of undistributed income	16,162	19,183
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,435	3,435
Effect of dilutive stock options (in shares)	5	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,440	3,435
Basic Earnings Per Share (in dollars per share)	$ 4.71	$ 5.58
Diluted Earnings Per Share (in dollars per share)	$ 4.70	$ 5.58
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	470	558
Allocation of net income:
Dividends	0	0
Allocation of undistributed income	$ 470	$ 558
Denominator:
Basic earnings per share - weighted-average shares (in shares)	200	200
Effect of dilutive stock options (in shares)	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	200	200
Basic Earnings Per Share (in dollars per share)	$ 2.35	$ 2.79
Diluted Earnings Per Share (in dollars per share)	$ 2.35	$ 2.79