Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues	$ 149,407	$ 190,985	$ 127,609	$ 196,662	$ 159,875	$ 92,354	$ 140,151	$ 180,367	$ 664,662	$ 572,747	$ 575,992
Earnings (loss)	25,249	24,322	23,246	19,741	4,362	19,265	13,790	18,211	92,558	55,627	72,897
Common Class B [Member]
Earnings (loss)									2,618	1,574	2,065
Basic Earnings Per Share (in dollars per share)	$ 3.57	$ 3.44	$ 3.29	$ 2.79	$ 0.62	$ 2.73	$ 1.95	$ 2.58	$ 13.09	$ 7.87	$ 10.33
Diluted Earnings Per Share (in dollars per share)	$ 3.57	$ 3.44	$ 3.29	$ 2.79	$ 0.62	$ 2.73	$ 1.95	$ 2.58	$ 13.09	$ 7.87	$ 10.33
Common Class A [Member]
Earnings (loss)									$ 89,940	$ 54,053	$ 70,832
Basic Earnings Per Share (in dollars per share)	$ 7.14	$ 6.88	$ 6.58	$ 5.58	$ 1.23	$ 5.45	$ 3.90	$ 5.16	$ 26.19	$ 15.74	$ 20.67
Diluted Earnings Per Share (in dollars per share)	$ 7.14	$ 6.88	$ 6.58	$ 5.58	$ 1.23	$ 5.45	$ 3.90	$ 5.15	$ 26.19	$ 15.73	$ 20.61