Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 28,858	$ 29,183	$ 50,184	$ 49,009
Undistributed income	28,858	29,183	50,184	49,009
Allocation of net income:
Allocation of undistributed income	28,858	29,183	50,184	49,009
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	28,042	28,358	48,765	47,623
Dividends	0	0	0	0
Undistributed income	28,042	28,358	48,765	47,623
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	$ 28,042	$ 28,358	$ 48,765	$ 47,623
Denominator:
Basic earnings per share - weighted-average shares (in shares)	3,436	3,435	3,436	3,435
Effect of dilutive stock options (in shares)	1	2	1	2
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)	3,437	3,437	3,437	3,437
Basic Earnings Per Share (in dollars per share)	$ 8.16	$ 8.26	$ 14.19	$ 13.86
Diluted Earnings Per Share (in dollars per share)	$ 8.16	$ 8.25	$ 14.19	$ 13.86
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 816	$ 825	$ 1,419	$ 1,386
Dividends	0	0	0	0
Undistributed income	816	825	1,419	1,386
Allocation of net income:
Dividends	0	0	0	0
Allocation of undistributed income	$ 816	$ 825	$ 1,419	$ 1,386
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)	$ 4.08	$ 4.13	$ 7.10	$ 6.93
Diluted Earnings Per Share (in dollars per share)	$ 4.08	$ 4.13	$ 7.10	$ 6.93