Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income		$ 16,221	$ 28,858	$ 21,326	$ 28,113	$ 28,466	$ 29,183	$ 19,826	$ 98,396	$ 105,588	$ 96,247
Allocation of net income:
Allocation of undistributed income									$ 97,123	$ 104,315	94,974
Denominator:
Antidilutive securities excluded from computation of EPS (shares)									21,000	22,200
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									$ 95,613	$ 102,602	93,524
Dividends									(1,237)	(1,237)	(1,237)
Allocation of net income:
Dividends									1,237	1,237	1,237
Allocation of undistributed income									$ 94,376	$ 101,365	$ 92,287
Denominator:
Basic earnings per share - weighted-average shares (in shares)									3,436,000	3,436,000	3,435,000
Effect of dilutive stock options (in shares)									1,000	1,000	4,000
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									3,437,000	3,437,000	3,439,000
Basic Earnings Per Share (in dollars per share)	$ 9.05	$ 4.59	$ 8.16	$ 6.03	$ 7.95	$ 8.05	$ 8.26	$ 5.61	$ 27.83	$ 29.87	$ 27.23
Diluted Earnings Per Share (in dollars per share)	9.04	4.59	8.16	6.03	7.94	8.05	8.25	5.61	$ 27.82	$ 29.85	$ 27.19
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									$ 2,783	$ 2,986	$ 2,723
Dividends									(36)	(36)	(36)
Allocation of net income:
Dividends									36	36	36
Allocation of undistributed income									$ 2,747	$ 2,950	$ 2,687
Denominator:
Basic earnings per share - weighted-average shares (in shares)									200,000	200,000	200,000
Effect of dilutive stock options (in shares)									0	0	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									200,000	200,000	200,000
Basic Earnings Per Share (in dollars per share)	4.52	2.29	4.08	3.02	3.98	4.03	4.13	2.80	$ 13.91	$ 14.93	$ 13.61
Diluted Earnings Per Share (in dollars per share)	$ 4.52	$ 2.29	$ 4.08	$ 3.02	$ 3.98	$ 4.03	$ 4.13	$ 2.80	$ 13.91	$ 14.93	$ 13.61