Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per 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
Revenues	$ 184,315	$ 159,620	$ 206,841	$ 158,003	$ 258,184	$ 188,192	$ 182,859	$ 231,032	$ 708,780	$ 860,267	$ 664,662
Earnings (loss)	28,113	28,466	29,183	19,826	27,842	24,782	26,991	16,632	105,588	96,247	92,558
Common Class B [Member]
Earnings (loss)									2,986	2,723	2,618
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
Common Class A [Member]
Earnings (loss)									$ 102,602	$ 93,524	$ 89,940
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