Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 28,113	$ 28,466	$ 29,183	$ 19,826	$ 27,842	$ 24,782	$ 26,991	$ 16,632	$ 105,588	$ 96,247	$ 92,558
Allocation of net income:
Allocation of undistributed income									104,315	94,974	91,285
Denominator:
Antidilutive securities excluded from computation of EPS (shares)									22,200	27,400
Class A [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									102,602	93,524	89,940
Dividends									(1,237)	(1,237)	(1,237)
Allocation of net income:
Dividends									1,237	1,237	1,237
Allocation of undistributed income									101,365	92,287	88,703
Denominator:
Basic earnings per share - weighted-average shares (in shares)									3,436,000	3,435,000	3,435,000
Effect of dilutive stock options (in shares)									1,000	4,000	0
Diluted earnings per share - adjusted weighted-average shares for assumed conversions (in shares)									3,437,000	3,439,000	3,435,000
Basic Earnings Per Share (in dollars per share)	$ 7.95	$ 8.05	$ 8.26	$ 5.61	$ 7.88	$ 7.01	$ 7.64	$ 4.71	$ 29.87	$ 27.23	$ 26.19
Diluted Earnings Per Share (in dollars per share)	$ 7.94	$ 8.05	$ 8.25	$ 5.61	$ 7.87	$ 7.00	$ 7.62	$ 4.70	$ 29.85	$ 27.19	$ 26.19
Class B [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income									2,986	2,723	2,618
Dividends									(36)	(36)	(36)
Allocation of net income:
Dividends									36	36	36
Allocation of undistributed income									$ 2,950	$ 2,687	$ 2,582
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)	$ 3.98	$ 4.03	$ 4.13	$ 2.80	$ 3.94	$ 3.51	$ 3.82	$ 2.35	$ 14.93	$ 13.61	$ 13.09
Diluted Earnings Per Share (in dollars per share)	$ 3.98	$ 4.03	$ 4.13	$ 2.80	$ 3.94	$ 3.51	$ 3.82	$ 2.35	$ 14.93	$ 13.61	$ 13.09