Earnings Per Share (Computation Of Weighted Average Number Of Common Shares) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Distributed earnings									$ 30,872	$ 0	$ 0
Undistributed earnings									102,434	125,096	111,764
Net income	30,475	34,486	36,442	31,903	34,012	31,376	32,167	27,541	133,306	125,096	111,764
Class A Common Stock [Member]
Numerator for basic net income common stock									85,172	78,921	68,837
Numerator for diluted net income common stock									85,323	79,183	69,192
Basic weighted average common shares	23,578	23,513	23,357	23,206	23,082	23,010	22,872	22,415	23,415	22,847	21,980
Potential exercise of stock options									115	207	298
Diluted weighted average common shares outstanding	23,643	23,607	23,510	23,357	23,251	23,171	23,126	22,727	23,530	23,054	22,278
Class B Common Stock [Member]
Numerator for basic net income common stock									48,134	46,175	42,927
Numerator for diluted net income common stock									$ 47,983	$ 45,913	$ 42,572
Basic weighted average common shares	13,193	13,193	13,271	13,275	13,275	13,276	13,317	13,605	13,233	13,367	13,707
Potential exercise of stock options									0	0	0
Diluted weighted average common shares outstanding	13,193	13,193	13,271	13,275	13,275	13,276	13,317	13,605	13,233	13,367	13,707