Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 21,326	$ 19,826
Undistributed income	21,326	19,826
Allocation of net income:
Allocation of undistributed income	21,326	19,826
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	20,723	19,265
Dividends	0	0
Undistributed income	20,723	19,265
Allocation of net income:
Dividends	0	0
Allocation of undistributed income	20,723	19,265
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,436	3,435
Effect of dilutive stock options (in shares)	2	2
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,438	3,437
Basic Earnings Per Share (in dollars per share)	$ 6.03	$ 5.61
Diluted Earnings Per Share (in dollars per share)	$ 6.03	$ 5.61
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	603	561
Dividends	0	0
Undistributed income	603	561
Allocation of net income:
Dividends	0	0
Allocation of undistributed income	$ 603	$ 561
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)	$ 3.02	$ 2.80
Diluted Earnings Per Share (in dollars per share)	$ 3.02	$ 2.80