Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 29,183	$ 26,991	$ 49,009	$ 43,623
Undistributed income	29,183	26,991	49,009	43,623
Allocation of net income:
Allocation of undistributed income	29,183	26,991	49,009	43,623
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	28,358	26,227	47,623	42,389
Dividends	0	0	0	0
Undistributed income	28,358	26,227	47,623	42,389
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	28,358	26,227	47,623	42,389
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)	2	7	2	6
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,437	3,442	3,437	3,441
Basic Earnings Per Share (in dollars per share)	$ 8.26	$ 7.64	$ 13.86	$ 12.34
Diluted Earnings Per Share (in dollars per share)	$ 8.25	$ 7.62	$ 13.86	$ 12.32
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	825	764	1,386	1,234
Dividends	0	0	0	0
Undistributed income	825	764	1,386	1,234
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	$ 825	$ 764	$ 1,386	$ 1,234
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)	$ 4.13	$ 3.82	$ 6.93	$ 6.17
Diluted Earnings Per Share (in dollars per share)	$ 4.13	$ 3.82	$ 6.93	$ 6.17